im not familiar with Slog2 system, so, i wouldnt know how to get around it. sorry. at first i had a assumption that you doing IR video, because it has very similar case.

to be honest, you're going to do a lot testing on a lot filter before you get to the right ND filter. i lack of the experience to advise how the filter will react and cause what kind of color shift on high iso video when stacked more then 2 ND filters. iso itself will struggle and might read the wrong color. this doesnt worry me, what really worries me is when the stop are so high, the AF refuse to work (even contrast detect)... and the feed to LCD may turn pitch black too. not sure with A7s, but the last best sensor was the sony 24mp turn black display at 10stop(lee big stopper) facing the sun (sunset shot)
if its on 24mm, i dont know which 24mm its going to be, i assumed its not sony since sony doesnt has any F1.4 24mm. the step up filter doesnt do any good. because each lens is design differently and the requirement of filter clearance are diff. but please do consider square/holder system. There are holder system are design to avoid vignetting for UWA(with few condition too).

other case is, vignetting is a big subject, you might avoided filter vignetting entirely(not sure the correct word to use it), but u cant avoid a very natural soft vignetting that cause by F1.4, even without filter. so, you will required to do some post processing towards it, or, leave it alone since natural vignetting is kinda nice too, it helps to pop the subject in center.

lets Assume you are using 35mm, i will say you are safe from filter vignetting, but the natural F1.4 vignetting will still occur.

*correction, when we/i mention filter vignetting, i'm assuming a hard black mark on 4 side of the image even at F8. meaning the filter used prologue into the filter clearance. not the natural vignetting cause by the barrel itself.
before getting out of hand, lets use the word "screw on filter" circular may lead to diff misunderstanding type of 'effect' filter.
The answer would be simple, some branded holder filter system are designed to work with UWA. hence, its pretty much 'safe' to use. for example, the LEE holder, the adapter ring has 2 type, UWA adapter and standard adapter. the UWA adapter will bring/pull/push/???? the holder backwards, giving more clearance at the front. (try google it)
The crisscross effect happen in UWA and at certain degree of the VND filter (turn-able), usually happens when you overturn till the max at 9 stops. i wouldnt recommend it for your usage, because it will introduce a lot of new possible issue/problems along the way. but if you insist, Do Not Buy ANY Brands of VND.... except, LightCraft Workshop. LCW. dato lee chong wei filter. hehehehe joke aside. it is known as the BEST and mother of VND. very pricey thou. its like buying two B+W KSM CPL MRC Nano filter. close to 1k for the 77mm pro version.  thinking to upgrade my version I to version II for a long long time, but pocket empty... and btw, LCW has came out with a 100mm square VND (design for Lee square CPL holder), and i have totally no idea how it works... i suspect its a linear PL.

sadly to tell you,... the limitation of square holder to work on UWA is maximun of stackable filter are TWO...
color case happen when u stack certain resin filter aka Square type and certain brands... no matter its long or short exposure, the color cast will be there, its the issue of that chemical used, the ability to reduce visible light may be incomplete leaving certain type of light escape through. this you really have to try it out yourself. so you gotta make many photographer friends to see who has the b+w ND1000 and borrow to test it.
btw, there are many terms to ND, some put 10stop or ND1000, or 3.0. you will need to read up a little how they catagories them. the 3.0 actually means 10 stop, they calculate by 0.3 as 1 stop, hence 3.0 is 10 stop. the 1000x also mean 10 stop, but im not sure how they calculate it.

.... now.... come to settle your problem, F1.4@3200 with 1/50 at bright day light. honestly... stacking 2 ND filter isnt such great idea as it will create a lot limitation especially when using more then 10stops. my best advise, may sound very absurd so i apologies first, but, you can use a single 10stop, and not sure if its possible, coz i dont know its a controlled video environment or chasing around actual day wedding shoot with is not within your power to control, you can try choosing the right time/moment of the day when the sun isnt that harsh, eg early morning. but if its uncontrollable then sorry i dont know how to advise you. but i hope my explanation will clear up some doubt and question you had.

btw, it is okay for you to explain what/how slog2 works? maybe we can refine a solution from there.

Actually, for your issue, this is what I believe a variable ND is designed to take care of.

Stacking more filters in front will eventually degrade your final output anyway, no matter how expensive they are.

You just have to work around the limitations in the best possible manner.

so, is it possible to use neutral color profile(reduce contrast, saturation to flat), setting dro to level 3 (to lower contrast even more) and adjust contrast/saturation accordingly? and since at such low iso at 100, it shouldnt be any noise issue by dro gain? i think it might work since if you are doing color grading later, it should cause any big issue since its shot in low iso.

edit: then u wont have issue needing a 12stop filters.... a 10 stop will do just fine? then u might only need a 6 stop ND, then your AF might just work without problems and LCD feed maybe viewable?

this is the part im getting confuse. when u mention native iso3200, it means the highest native is 3200 right? because below that, those 1600 down to 200 is considered native iso too. right? need albert to verify this...

I had made a decision to buy A6000 in september (waiting price drop a little first), but then Sony launch A5100, really mess me up

A5100 pro :
1. 180" articulated screen for selfie
2. TOUCHSCREEN !!! (very important)
3. slightly cheaper at 700 usd (not important)

Con:
1. no EVF
2. slightly slower FPS (not really important)
3. smaller size

Why damn SONY always launching more camera body than lens ?!

I had made a decision to buy A6000 in september (waiting price drop a little first), but then Sony launch A5100, really mess me up

A5100 pro :
1. 180" articulated screen for selfie
2. TOUCHSCREEN !!! (very important)
3. slightly cheaper at 700 usd (not important)

Con:
1. no EVF
2. slightly slower FPS (not really important)
3. smaller size

Why damn SONY always launching more camera body than lens ?!

A6k vs A5100
Evf, when need time if don't have, will regret
Hotshoe, when need time if don't have, will regret.
Touch screen? Fun for first 3 month, after that never border coz can't use during the phase detect mode af-c + multiaf. Turn the wheel dail scroll faster in review mode the touch scroll.
Selfie lcd? Settle with NFC remote app.

Others consider not that influenced to purchase decision.

Hope my info help you make up your mind kekekeke

I had made a decision to buy A6000 in september (waiting price drop a little first), but then Sony launch A5100, really mess me up

A5100 pro :
1. 180" articulated screen for selfie
2. TOUCHSCREEN !!! (very important)
3. slightly cheaper at 700 usd (not important)

Con:
1. no EVF
2. slightly slower FPS (not really important)
3. smaller size

Why damn SONY always launching more camera body than lens ?!

Does anyone here own the RX100 M3? Seems like theres good reviews all over. MIGHT consider moving to that since I find my A77 abit too bulky. But I know that I'll definitely miss my A77.

hotshoe is not important for me,will never buy flash.
touch screen important cause i read lots of review complaining A6000 hard to set focus point (in 179 point) and wifi/nfc connection need password, need to scroll till u die...
Selfie lcd - use hp need few steps to connect n load vs just flip the screen, and im a solo backpacker,so selfie lcd kind of important for me...

So in the end , EVF vs Touchscreen+Selfie LCD, im more towards touchscreen and fully circulated screen

but A5100 start selling in mid September, dont know when wil reach malaysia and i need it before end of september... so in the end i will buy A6000 i guess 

hahaha u got your answer of what u need already  you wont regret with A6k, the flash u say wont buy now,.... eventually 2-3 month later u will start asking here which flash is good for your A6k hahaha its been a repeat pattern in Sony thread.

but i need to correct something for u, i dont know where you read the selectable 179 point.  the 179 points is not selectable. you only can select the 25 contrast AF point, the 179 point will asssit the 25 point from behind. no such thing as scroll till you die. A6k user please verify this. as far as i know (from the prototype model that i was testing...)
the 179 assist af point only work in 2 AF area mode, both require to be in AF-C. one is the Wide area AF, one is Zone area AF. the zone is group into 9 squares... and its damn easy to navigate in the 5x5 box area. this was my finding during the test. the 179 will assist in the zone 9 square mode.

hahaha u got your answer of what u need already  you wont regret with A6k, the flash u say wont buy now,.... eventually 2-3 month later u will start asking here which flash is good for your A6k hahaha its been a repeat pattern in Sony thread.

but i need to correct something for u, i dont know where you read the selectable 179 point.  the 179 points is not selectable. you only can select the 25 contrast AF point, the 179 point will asssit the 25 point from behind. no such thing as scroll till you die. A6k user please verify this. as far as i know (from the prototype model that i was testing...)
the 179 assist af point only work in 2 AF area mode, both require to be in AF-C. one is the Wide area AF, one is Zone area AF. the zone is group into 9 squares... and its damn easy to navigate in the 5x5 box area. this was my finding during the test. the 179 will assist in the zone 9 square mode.

Sony Malaysia shared the Alpha Super Workshop info again on fb page an hour ago, so I think there are seats still available.

Those who yet to register fast grab lo!
https://www.sony.com.my/AlphaSuperWorkshop/