"Cloud provider: AWS originally, and with so much custom infrastructure built on its APIs, unlikely to ever change."

Who cares? It literally doesn't matter unless you are doing something very, very specific with ML. As far as line-of-business projects go, every service you need has a direct equivalent. Also, NO ONE EVER SWITCHES CLOUDS, this is a TERRAFORM MYTH. Terraform being USEFUL in Cloud transition is a MYTH. I have direct experience with this where a corp started competing with Amazon (once Amazon went into groceries) and had to move to Azure. This is the ONLY instance where it made sense.

"The underlying database: Mongo originally begot more Mongo, but switching any database anywhere is unusual."

RDS is best 90% of the time.

Other than that, just keep it modular. Definitely no MVC front ends married to the BE - API always.