Car tuning is like playing Golf, the never ending quest for consistency.
There are so many variables involved here, it's hard to pinpoint it to something specific. But bear in mind the mods you have already applied have stiffened up the chassis considerably, so will naturally make the car more twitchy on worn out roads. Every component you stiffen up under there, moves the stresses onto the next weakest link, so.....
...check the following for play:
Front wheel bearings
Inner tie rod ends
Outer tie rod ends
Steering rack - play between rack and pinion.
Steering rack UJ
The tiniest amount of play in any of those components is amplified massively over the diameter of the steering wheel. For example, 0.5mm play at the rack/TRE/UJ = ~1cm of free play at the steering wheel.
How low is the car? Too much lowering can also cause problems.
What tyres have you got? Golfs are picky with them in my experience. For example, F1s = vague when half worn, PS4 = much better self centering and off centre feel.
You've done some bushes, but you might benefit from doing ALL of them, and not necessarily poly. A lot of folk automatically go to poly, not realizing a new OE rubber bush is considerably stiffer than a 10 year old worn out squidgy one, and more compliant, and cheaper.
Try dialing in a bit of front toe. They don't run any as standard, which can cause a bit of twitchiness in the steering wheel.
Even the standard suspension is twitchy, so you face a big battle getting aftermarket suspension to feel consistent!