Composr does not officially support flat-file installs. There is the XML driver, but it is only available when using Composr from Git (it was intended for development testing only and has a lot of nuances).
In fact, the XML driver does not work at all in v11, and I have no plans right now to fix it (costs outweigh benefits).
However, I'd suggest adding a feature request for something like SQLite support, which can easily be done since PHP supports it natively.
I'm adding SQLite 3 to v11 beta9. But it will only have basic support. It won't work well for major upgrades at this time because it doesn't support changing column properties (without creating a new table and migrating everything over).
I'm mainly doing this for additional dev testing capabilities. But it will work for admins of simple sites who want just a flat file for the database.
Cool, not sure how useful it will be for my use case. It seems the galleries table takes up a lot of my database and that keeps on growing, hence why I tried the option in v10. Sadly, v10 did not deliver.
System message - Issue updated
In fact, the XML driver does not work at all in v11, and I have no plans right now to fix it (costs outweigh benefits).
However, I'd suggest adding a feature request for something like SQLite support, which can easily be done since PHP supports it natively.
I'm mainly doing this for additional dev testing capabilities. But it will work for admins of simple sites who want just a flat file for the database.