I'm assuming 2 things - 1. The data is accessed by people, so an 50ms of latency doesn't matter so much for most purposes. Writes/modifications are infrequent so even a global lock when writing would be ok. 2. You have about a few hundred GBs of data at most.

If thats the case, the technical problem doesn't really require anything novel. A simple SQL engine like sqlite works - its very common, most programmers know or can pick up SQL and its mature/stable/battle-tested. I would go with that.

For a more detailed answer, maybe you could bring in a programmer or consultant to take a look, although they'll probably charge you way more than its worth.