Anyone looked at DuckDB [1]? Like SQLite, it's an embedded relational database with a C API. In fact, it has a wrapper which presents the same API as SQLite [2]. But it's optimised for analytical rather than transactional work - a few huge queries rather than lots of little reads and writes.
[2] https://github.com/cwida/duckdb/tree/master/tools/sqlite3_ap...