Big Sur 11.4, when I removed Radeon VII and added 6900XT into the same slot (SLOT1), the TB3 card started freezing the system. I thought it was related to something else, but did a lot of troubleshooting trying different slots for TB3 card and it definitely was the card. Also thought it was WEG related, so I left it in there with the pikera and wegoff args, but that didn't help either. So I removed the TB3 card and WEG and everything is 100% stable now.
With the new 6900XT waterblock, I had to move the TB3 card down from Slot 2 (which my mobo recommends for TB3) into Slot 3. Instead of using the TB header again, I did the 3>5 Pin mod and removed that cable and it seemed to show up fine even after disabling TB in the BIOS. However the freeze continued on all slots.
This was the flashed firmware on the TB3 card
View attachment 521166
I'm just wondering if it's worth flashing and trying again. I haven't tested this card with 0.7.0 yet, but figured OC (freeze happened on 0.6.9) wouldn't make much of a difference for the TB3 freeze issue. I don't use TB3 a lot, but I do need it once in a while, but I have to get some new gear for the RPi to flash.