SQLite (also H2 and some other embedded SQL databases) can be used entirely in-memory, one can also drop an SQLite file on a RAM-hosted filesystem (tmpfs/ramdrive). You really can put everything into RAM (and still enjoy SQL) if you have enough, don't mind long cold-load and potential data loss.

I'd argue adding SQL into the mix makes it difficult to maintain, mixed-language codebases are almost by definition complex, and you get significant chafing when mixing a declarative language like SQL and OOP.

Since this is a no-update and no live-insert scenario we're talking about, it's fairly easy to produce code that is an order of magnitude faster than a DBMS, since they're not only primarily optimized for efficiently reading off disk (an in-memory hash table beats a B-tree every day of the week), they've got really unfortunate CPU cache characteristics, and additionally need to acquire read locks.