I'm not sure if you're just picky or discerning but it seems you can find a reason to exclude anything if all you do is look for reasons to exclude.

Why not just use SQLite with streaming replication? It should fit your bill.

Databases rarely have what you would define as real open source with real contributors because the nature of a database means you need one owner and that owner has to be picky and exclude things. Allowing the wrong commit into MariaDB could introduce regressions that no one even could imagine because the complexity of things.

Even when a database starts out the way you describe with good intentions in order for it to become a widely adopted product it has to be pulled in under a single umbrella to direct it and build it toward its vision. This puts it firmly in your fake open source camp.