I feel connection pooling is much more rare in SQLite libraries than it should be. I'm maintaining one implementation (sqlite_async for Dart), but feel like this should be the standard for all languages with async/await support.

I completely agree. But I simply have no reference / good-practice implementations to take inspiration from. I'd be more than willing to have an Elixir FFI bridge to a Rust library (and write both in the process) that actually make full use of parallelism to fully utilize SQLite's strengths but again, I got nowhere to steal from. :) Or I am not aware where to look.