Firebird can be embedded, although neither the database itself, nor the embedded mode are as popular as they once were.
It's a fully featured database though, with everything you expect from one, including actually working ALTER TABLEs.
DuckDB ? Strict by default and excellent for logs, telemetry, dashboard apps, etc.
From what I can tell, DuckDB is more focused on huge-scale data analysis than simple data persistence, so I’m not sure if it fits my use cases. Otherwise, it looks good.