At least with other instance types (m5,m6,t3) it was the case that the AMD processors were cheaper. As it turns out, this does not seem to be a general rule.
It seems like performance wise the AMD processors are (in certain workloads) quite a bit faster than their Intel equivalent: https://www.phoronix.com/review/aws-m7a-ec2-benchmarks/2 (in later pages it seems to be a little bit more mixed)
I'm honestly not sure if thats a great idea, but this might be a possible way to do one account per DB/backend/frontend in a somewhat sane way.