> when you pay for a hot standby you can't use it as a read only replica (true for AWS
I'm not sure what you mean here. At least for MySQL you can have an instance configured as replica + read-only and used for reads. Aurora makes that automatic / transparent too with a separate read endpoint.
A hot standby is not a read replica. It's a set of servers running in a different Availability zone mirroring current prod, that is configured to automatically fail over to if the primary is offline. It's been a few years since I personally set this up in AWS, but at the time, those servers were completely unavailable to me, and basically doubled the cost of my production servers.
The fact that a hot standby is usually in some sort of read-replica state prior to failing over is a technical detail that AWS sort of tries to abstract away I think.
I'm not sure what you mean here. At least for MySQL you can have an instance configured as replica + read-only and used for reads. Aurora makes that automatic / transparent too with a separate read endpoint.