O M G
Dude, you are going to kill those cards… running the GDDR6 over 100C is a death sentence.
The hashrate is dropping because the RAM is throttling itself in an attempt to protect itself from damage.
You will need to fix the cooling on those cards… I have two MSI RX 5700 and I took the plastic backplates off the cards and put fans on the back of each card blowing on the PCB directly. This dropped the temps by 10C+. The plastic backplates actually inhibit cooling of the PCB because they block airflow and are NOT heatsinks (plastic is an insulator).
I also took the cards apart and put a small amount of thermal paste on each of the RAM thermal pads (both sides) to improve the heat transfer.
The backplate removal requires taking the heatsink off, so you might as well put some paste on each of the VRAM thermal pads while you have the heatsink off.
The backplate is held on by a few screws that are only accessible from front-side of the PCB.