If you are good in linux I believe you can do vega undervolting all ready.
https://github.com/RadeonOpenCompute/ROCm/issues/361

I failed to make tdxminer running under this Linux build but undervolting works.

Can be also usefull. https://phoronix.com/scan.php?page=news_item&px=AMD-ROCm-1.8.2-Beta