I've made some progress after trying different ssds.
First I realized a grave mistake: I did not remove the film from the thermal pad before installing what would be my main drive. I removed it, hoped all was fixed, tested with a game and it still crashed. I'm guessing since I tested so soon after a previous crash, the m.2 was still too hot.
I removed both m.2s (inland and 990) then installed the old m.2 (970) from my previous build. Although it's not optimized since the win10 install on it was used with a different motherboard and parts, I could still run steam. The m.2 (inland) that was in the rear slot contained most of my steam library, I took that and put it in a m.2 enclosure and connected it via USB C. Still on this Win10 drive, I added the steam library from this external drive so I could play a game and see if it crashes. It did not crash. (yay!)
So I think it was the main m.2 (990) heating up that was the culprit. I did a fresh install of Win11 on the problem m.2, installed a game on it, played a long enough session and it didn't crash. So at this point it seems the issue is fixed.
I then played a more demanding game, and it did end up crashing but not as quickly as before. I feel as though if I were to turn up the GPU fans I'll be fine, just like I used to in my last build. If this is the case, I'm happy. From here I'm going to look at still improving my build. I still have a new 645LT to install, I want to replace the gen3 riser with a gen4 one, I need to figure out undervolting, and then also ensure power draw for the CPU is properly configured. I may down the line try the noctua fan with vent route to see if it the temp/noise trade off is worth it. I appreciate everyone's help! Legion especially!
EDIT: It crashed at desktop. 990 M.2 permanently damaged from repeated overheating maybe?
EDIT2: Did more testing, issue isn't fixed. Crashes in-game and occasionally on desktop no matter which m.2 drive I'm running off of. I'll have to set aside a day to do the cardboard bench pc testing.
EDIT3: It was the riser cable. Connected gpu straight to the board, it worked without a hitch.
