1. Do you really have to go for Nikon Z6ii ?
I would suggest you go for Sony A7iii or Canon Eos R or even the Z6 mark 1 to save yourself some money since you're doing studio work. Would you consider APSC bodies? The Fujis are great options such as the XT3 and XT4

I'm recommending this so that you can cut down on the spending on the camera body and can spend more on lighting and softboxes.
2. For studio lighting equipments, now the in thing is "Godox"

I would recommend you get i) Godox Ad200 Pro, they are the 200watts second flashes, ultra portrable and powerful for the size.
                                        ii) Godox X Pro or X2T triggers
                                        iii) Triopo 120cm Softbox (for your main lighting)
                                        iv) Godox stripbox 30 X 150cm (for side lighting)
                                        v) A reflector
                                          vi) 4~5 lightstand. (1 for main light, 1 for side light, 2 for your backdrop and 1 for your reflector)
                                        vii) Backdrops (they are ultra important to make your portrait look dramatic
This are some of my works incase you wonder

DSCF0170 by Apa Macam, on Flickr

DSCF2351 by Apa Macam, on Flickr

DSCF3288 by Apa Macam, on Flickr

I think Z5 is good enough. Either way, you won't go wrong.

I got 5 lightstands, why ? 2 for the backdrops, 1 for front light and 1 for backlight and 1 for the reflector. Having said that, you can at least cut down 2 from that setup. If you go with T stand, then u only need 1 instead of 2 for backdrop. If you don't use the reflector, then you can cut away another one. It really depends on yourself.

I have 2 Godox Ad200 (non pro version) which I got for RM780 and 750 respectively. So, both of them cost me about the price of an AD300, which is quite affordable. I also have the v860ii and the tt685. If I have the option, I will always go for the Ad200 for the main light as well as for the hair light.

  Photography is a form of art, there is no right or wrong. If that's the effect you want, why not? AS for me, I want my subject to have all the attention of the viewer, so for me, shadows are important to. As such,  I don't think I want to have so much fill lights. It's not my style, at least not now.