Normal OEM or OE replacement shock has no gas fill valve. Refill gas means full rebuild, tear down to replace the oil, rubber seals altogether.

There are shops offering this service, but not worth the effort/cost for most OEM shocks and aftermarket non-adjustable performance shocks like Bilstein B6, B8.
Besides, most OEM shocks are twin tubes. The gas pressure is very low, usually only 2-3 bars. Some even 1 bar same as atmospheric pressure, especially for offroad vehicle.

If you have to ask this question, probably belongs in not worth the effort/cost bucket.

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Good adjustable shocks are designed to serviceable with gas fill valve.
Still a good reputable shock even high pressure filled monotube shock rarely have gas pressure issue from street use for at least good 8-10years, when it is time just send for full rebuild.

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Monotube Shock is the one that have high pressure 10 bars+ or 150psi+, racing use 200-300psi is not uncommon.

At 100-150psi range, the upward force is only about 20kg, so even with monotube, in a good tuning car weight is supported by the spring, not gas pressure.
Shock job is control rate of spring oscillation, speed of compression/rebound. Maximum pitch and body roll are primarily still job of spring.
Gas pressure key job is to prevent foaming or fancy word cavitation.

** let's ignore motion ratio effect of lower control arm length for simplicity.
Unless one is running very soft OEM spring at stock car height eg:. 2k spring rate, 20kg yield 10mm.
10mm is huge for Sport sedan, sport car, tuned car but on SUV or stock B Segment it is unnoticeable for most people.

Then one should also question why pair such a soft spring with a performance shock, the natural frequency will be all out.
Even a light weight Miata runs 4k - 8k spring rate depending on the tuning.
Random example with a common setup eg:. 10k spring rate, 20kg yield only 2mm.

Compare this with compression from full length supported by spring, negligible.
Easy to talk fancy oh my gas shock support car height, practically it only affect one ant length More likely his/her mechanic did a shabby job, not properly release the bushing preload during installation, messing with the heigh and stiffness.

My personal experience Ohlins DFV (monoshock) is insanely stiff, able to control spring 2-3 times stiffer than OEM and yet surprisingly comfortable.
I couldn't full compress with my body weight at stiffest. Even at softest setting was very difficult to compress, couldn't do it without mounting plate.

At 10 clicks (1 stiffest, 22 softest), I could literally felt the white lines. Currently running 16 clicks, a comfort setup.

Thank you to just prove my points. OEM shocks are not designed to be serviceable.

They are factory sealed, and as you said requires intrusion tools to hack vs reputable aftermarket adjustable shock that can unscrew to expose the valve gracefully, unscrew to dismantle gracefully.
Also replenish gas from the top, meaning yours are twintube OEM. Not the high pressure monotube where gas chamber is either at the bottom or separate canister.
Those low pressure gas don't exert enough force to cause noticeable ride height.

FYI - when people said gas shock, it is exclusively refer to shock with high pressure gas chamber that separate gas from fluid. Mostly monotube, or twintube with external high pressure gas chamber (shock body is 100% fluild no gas), like Ohlins TTX or selected KW Competition.

I don't doubt you did your service, whatever method you sworn by.
I believe fill gas on low pressure twintube OEM shock is not an meaningful service nor upgrade. Also the cost of most OE shocks don't justify for a full rebuild, might as well don't do anything until change a new one.

I can't not help you. Suspension is more like art than science.

What one complains can be desirable effect for another person.

Anyway some 2 cents here.
You mentioned you change mounting and bushing. If your mechanic tightening everything when car is in the air, then you need to find a different mechanic.
Find a reputable mechanic that can perform proper bushing preload release. Get the mechanic to ride with you together.
The shop should be 2 types of lift = Lift from the car body, lift from the wheel (or "longkang" at alignment shop also works).

Bushing preload release is most important step during installation (anything involved unscrew the suspension control arms)
1) Before dismantle, measure car height at all 4 corners when car is on the ground, write them down (from center of the wheel hub to the fender).
2) Lift the the car and remove the parts to be replace.
3) During installation, use a jack to life the lower control arm such that the wheel hub is at same distance from fender using measurement from step 1 (+/- 10mm, don't have to super precise, step 7 will take care of the precision).
4) Screw the parts, but do not fully tighten.
5) Drop the car to ground, drive forward and backward so that the car return to natural resting height.
6) Lift the car with lift that support the wheel, not the car body. Or drive to car alignment machine with "longkang".
7) Loosen all the control arm bushing, and tighten them to the factory OEM torque spec.

Additional not applicable to you.
8) If change sport spring, coilover that significantly lower the car 20-30mm+, need to loosen all the control arm and retighten when car wheel support the car weight. Many of them are difficult to access, so need a mechanical plate to install on the wheel hub instead of actual wheel for this exercise. 99% of mechanic don't have this, need to find tuning shop.

9) For commercial truck, if majority of time is fully loaded, we can also perform the bushing preload (purposely load, instead of release), when the truck if fully loaded. So that when truck is empty, the bushings are twisted, but closest to natural load free position when the truck is loaded. This prolong the bushing service life.

If didn't perform the steps above, the bushing are already twisting when car is resting on the ground. Not uncommon for the twisted bushing to add 10mm+ to car ride height.
The twisted bushings are adding stiffness on top of the car spring. Some more unevenly, adding to random harshness, bumpiness.

Also the new bushings life will be significantly reduced. Image resting already twisted, during bump or ferry people, the bushing are twisted even more than designed if installed properly.

New bushing, new shocks take few days to break-in.
Mounting and bushing should not affect steering, not sure what you mean steering bumpy.

If the car is lowered, it automatically add additional camber for most road car, that will cause steering to weight heavier.
Higher castor angle also can add weight to steering, and cause steering self return to center faster.

Both camber and castor are non adjustable during alignment for a normal road car without modification.

This post has been edited by constant_weight: Yesterday, 11:51 PM

Depends on where you are.

Most large shop with at least 4-5 lifts are capable of doing it.

The single shop lot type, mostly don't care. The faster your bushing rotted, the better, it is business opportunity.

Your case the bushing already installed, you can go to your trusted tyre shop's alignment bay.
Ask them to loosen all the control arms bushing bolt/nuts, give your car some good shake and tighten again, remember the wheel supporting the car weight, not in the air.

Especially you mentioned car uneven after new mounting and bushing replacement. I smell both side bushings are different tension.

This maybe not be the root cause, but it is the most relatable to your recent replacement and cheapest work with no parts replacement.
Try this first if none of shock absorbers are leaking.

If the shock absorbers are leaking, replace them right away. During the shocks replacement, they need to loosen the bushing on control arms.
Ensure they perform the same bushing preload release steps after install new shocks.

This post has been edited by constant_weight: Today, 11:17 AM

I don't doubt your buddy has some intrusive/abrasive tools to make it works, then seal/weld/glue back the hole/scar/gaps from the process.
Totally trusted you my friend, totally understand.

I think that is not worth the effort for cheap OE shocks, and the effect on a mere 5 years old low pressure shocks is negligible or placebo.

90% of people, can't even tell when we mix 1x new shock with 3x 10 years old shock (as long as the 3 are in working condition not leaking oil)

Anywnay, as long as you happy, I said previously whatever you sworn by.
Respect your decision, maybe just no need exaggerate recommend useless claim low pressure gas of shock can support your car height, restore car height as solution to solve TS bumpiness issues.
Not a concern for 99% of people with few millimeters off.
Remaining 1% do what they sworn by good or bad, you and me included.

Even during significant gas leak the shock become softer, causing boat ride. Not stiffer, not bumpiness.
If TS car is so soft and crushing the bump stop, he no need to ask here, any mechanic can tell even the bawah pokok one.

12 years old shocks, any issues just replace new set. Straightforward.

This post has been edited by constant_weight: Today, 11:08 AM