I have a 6800xt so 6900xt should be pretty much the same.

You want to make a mining profile in the driver tuning just for mining.
With 6800xt i get 64.5-64.6mh/s.
I have core set to 1219 (actually runs 1200 or 1205).
Voltage 625mV (I will explain how lower)
Fast timing on, 2150mhz speed (actually runs 2138mhz). Some people have to select 2140 i think.
Uses 137w in miner software. Undervolt made it run 5c cooler too.

To change volts i used this video https://www.youtube.com/watch?v=N07GTV06_-Q
Original bios file from here https://www.techpowerup.com/vgabios/?architecture=AMD&manufacturer=Sapphire&model=RX+6800+XT&interface=&memType=&memSize=&since=
Morepowertools here https://www.igorslab.de/en/red-bios-editor-and-morepowertool-adjust-and-optimize-your-vbios-and-even-more-stable-overclocking-navi-unlimited/3/

Re: 3090, if your just using it for mining then maybe 2x 3070 is better option. There are other threads here discussing this, seems most prefer 2x 3070.