DuckDB does aim to be Postgres compatible from a SQL syntax perspective, but you are 100% correct that it is not optimized for individual transactions. I'm a huge advocate of DuckDB, but strongly consider your life choices if you want to use it as a transactional database.

is there something like duckdb but suited for transactional database while still being postgres compatible

It might sound confusing but I wish to genuinely have something of a simpler postgres (in sqlite?) and then later if need be, i could migrate to postgres I suppose.