So did you try putting the 3070 you had previously in it? That would rule out the 3090 being the issue. To be honest though it sounds like the psu gave up the ghost and took out the motherboard with it I've had that in the past with cheap psu's that literally went bang and nigh on took out the motherboard with it.
Without swapping out components its difficult to know for sure whats going on if you can beg steal or borrow spare bits to try out do so otherwise its one of those nightmare situations to deal with
Havent swapped to the 3070.....will do tomorrow as Ive run out of steam! Expect no difference as Im typing on the rig running the 3090 now.
Swapping components is the only way, throwing more money at it now as Ive ordered another x299 mb. By the end of this I'll have enough bits for a 3090 system and a 3070!!!