I wonder why the Firebird database is not mentioned more often in such threads. It is a somewhat lightweight RDBMS that still has many advanced features that SQLite may not (by design, of course). It may be positioned somewhere between SQLite and heavyweight databases like MySQL, PostgreSQL, Oracle, DB2, etc. I've used Firebird, though only lightly, with Python. It also has different (lighter/heavier) versions for different needs.
https://en.wikipedia.org/wiki/Firebird_(database_server)
https://firebirdsql.org/
A few excerpts from this page:
Get to know Firebird in 2 minutes:
https://www.firebirdnews.org/docs/fb2min.html
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Firebird is derived from Borland InterBase 6.0 source code. It is open source and has no dual license. Whether you need it for commercial or open source applications, it is totally FREE!
Firebird technology has been in use for 20 years, which makes it a very mature and stable product.
Don’t be fooled by the installer size! Firebird is a fully featured and powerful RDBMS. It can handle databases from just a few KB to many Gigabytes with good performance and almost free of maintenance!
Below is a list of some of the Firebird’s major features:
Full support of Stored Procedures and Triggers
Full ACID compliant transactions
Referential Integrity
Multi Generational Architecture
Very small footprint
Fully featured internal language for Stored Procedures and
Triggers (PSQL)
Support for External Functions (UDFs)
Little or no need for specialized DBAs
Almost no configuration needed - just install and start using!
Big community and lots of places where you can get free and good support
Optional single file embedded version - great to create CDROM catalogs, single user or evaluation versions of applications
Dozens of third party tools, including GUI administrative tools, replication tools, etc.
Careful writes - fast recovery, no need for transaction logs!
Many ways to access your database: native/API, dbExpress drivers, ODBC, OLEDB, .Net provider, JDBC native type 4 driver, Python module, PHP, Perl, etc.
Native support for all major operating systems, including Windows, Linux, Solaris, MacOS, HP-UX and FreeBSD.
Incremental Backups
64bits builds available
Full cursor implementation in PSQL
Monitoring tables
Connection and Transaction Triggers
Temporary Tables
TraceAPI - know what happens in your server
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Disclaimer: I'm not associated with them in any way. Just have used the product a bit, and think it is somewhat good, based on that.