Think all aidis have dmf now. Even my old a6 does.
Here's someone with a similar issue but there's is an auto. There problems also started after a remap. I notice their torque is less than yours but that'll partly be due to there's being an auto.
https://www.google.com/amp/s/www.audi-sport.net/xf/threads/audi-a6-40-tdi-c8-gearbox-vibration.431231/%3famp=1
I'd definitely recommend speaking to the tuning company.
If it is the dmf you might need to upgrade it with an aftermarket one that's stronger, do a solid single flywheel conversion and uprated clutch. Here's an upgraded one for an example, but notice when the upgraded one can't handle over 475nm of torque whereas you're running over 500nm on the stick set up.
https://clutchstop.co.uk/products/luk-dual-mass-flywheel-sachs-performance-clutch-kit-for-volkswagen-audi-seat-skoda-02q-6-speed-gearbox.html
I've heard people talk about this company for upgraded clutches etc. But I don't see the A6 c8, but might be worth emailing them:
https://clutch-specialists.co.uk/