Vega 56 and Sonnet 350
I've got a Vega 56 in an Akitio Node (400 W power supply). I also figured based on that research and power draw that it was fine. But it shuts down pretty readily when doing Heaven benchmarking. In Windows it shuts down occasionally in other benchmarking as well. On OS X I've played probably 10 hours of Total War: Warhammer 2 with it and it shut down once unfortunately. I'm now looking at if I can switch to the low power BIOS or do the under-volting. Otherwise I'll need to upgrade the PSU I guess.
I think undervolting would solve your problems.
Mac mini 2018 / i7 8700b / 32GB ram / 256GB ssd