We eventually realized that the "API server" was actually just a DBMS all along. And once that was realized, it was realized that a DBMS sitting beside another DMBS that deals with the exact same data is rather silly, which is now leading to:

    1. Let the clients connect to Postgres directly.
         - or -
    2. Cut Postgres out of the picture and double down on the homegrown DMBS.

Some are going the #1 route, others #2. Where #2 is opted, SQLite is a convenient engine on which to build upon. It may not be perfect but it is what we have. Keep in mind that this realization on a grand scale (I'm sure some noticed many years ago) is fairly recent, so there is a lot of experimenting going on to figure out what works and what doesn't.

It's the natural cycle of computing. What is old is new again.

(Replace Postgres with MySQL, MSSQL, Oracle, or other DBMS as you see fit.)