I’m after some help/advice as I feel I’m going round in circles with this issue.

I have a 2012 Audi A6 (C7) saloon 3.0 BiTdi which I’ve had for the last 6 months, when I purchased the car there was an issues with the brakes, the pads had been replaced all round but the front drivers side wasn’t fitted correctly and caused uneven braking.
I took it to a local garage who sorted the problem and advised that I run it for a few weeks to see if the pad wears evenly but I may need to change the discs and pads if the disc has warped.
The vibration was terrible and only got worse so I went ahead and had the front discs and pads replaced, this seemed to sort the problem for a few weeks and then the brake judder came back but not so severe.
this followed a break pad warning light on the dash which after checking out was actually the tears that needed changing now so I had the rear discs and pads changed too but this didn’t sort the judder/vibration out?
i have since had the car back in and the local has taking it for a drive and checked all the discs out and said they are all within tolerances and at the expected temperatures and would need to start stripping things to further investigate?
this to me seems like it could get costly looking for something they are not entirely sure what it is?

These are the things I’ve tried:
new discs pads all-round
wheels balanced

The car is running on 20” RS6 replica wheels which needed a 3mm spacer on the fronts but was advised that the wheel is still sat on the hub so this shouldn’t be causing the issue.

The vibration/judder is more noticeable with light braking and seems more evident at 40-30 mph when slowing down.

any more advice would be welcomed before I try and investigate it further.