I could be wrong, but the symptoms and solution you described would have fitted perfectly to a temperature issue. Symptom: Does not crash immediately, but takes a few seconds. In the menu, only your gpu is hammered to 100%, not your cpu. If your system fans are cpu temp controlled, this would also mean less airflow.
Solution: FPS limit, since it lowers temps significantly.
You would have to do a low enough stable undervolt to fix the issue, if it is core temp related (3d mark should help with testing stability before trying MSFS and fur mark is able to at least stress and heat up your gpu near msfs temps, while showing temp and clock). I assume you lowered clock and voltage as you sound reasonable tech savy?
If gpu temperature is really not the cause, maybe someone else can help/has an idea. I don‘t think it is the PSU. If your hot spot gpu temp does not go above 75C you should be fine, as long as you tested right.
Fun fact: With factory settings the manufacturer gave my GPU 1.2V. With 3d mark my GPU temp goes to about 75-80C. MSFS exceeds 90C core temp.