$100 for a broken 390X in 2021 are you mad?I'll give you the only real answer.
Put it on ebay "as is" "parts only" and some dummy will give you $100 for it.
Use this $100 to purchase another microwave.
it'll go for at least $200
OP, if u read this, I feel bad for ya and ppl are being mean, but yr GPU and mobo are done. The GPU 100% for sure and I would be surprised if the motherboard is salvageable.