This may not be the answer you're looking for, but...

Unless you have firm requirements for your system running on top of multiple database servers, then you need to consider simply ignoring that angle for now. Just work on one database, and worry about supporting multiple systems when you need to (not before you need to).

Alternatively, use an off the shelf framework to do all the abstraction for you.

I'm currently working on a new legacy system (sic - not even finished, but already causing trouble and planned to be replaced) that's massively complicated because of unnecessary and/or unused in-house groundwork/preparation/abstraction/just-in-case-isms. If only my predecessors had chosen to use an existing library, I might be able to recommend one. Hopefully, some other monks will have opinions.