Has this been suggested before? Optional support for a databases other than sqlite, especially for server (docker) installs. Mariadb, mysql are popular options amongst more advanced users (like myself) that self-host applications like Home Assistant, etc.
This might open the door to a whole new set of future capabilities, including perhaps concurrent access to the database from multiple instances of photostructure, or even new third-party capabilities to display library stats, etc. ( I understand these things could also be achieved through REST APIs)
It should certainly take care of several recurring issues with locks, slow fs (and workarounds like the force replica feature).
Just a thought…