Thought I would let you all know in case you have missed it. A few days ago Postgres support was finally merged into Sonarr dev branch (meaning 4.x version). I have already transitioned to it, so far it runs without issue
You can mostly follow the same instructions as for Radarr from here: https://wiki.servarr.com/radarr/postgres-setup
I used the following temporary docker container to do the conversion (obviously replace stuff you need to):
docker run --rm -v Route\to\sonarr.db:/sonarr.db --network=host dimitri/pgloader pgloader --debug --verbose --with "quote identifiers" --with "data only" "sqlite://sonarr.db" "postgresql://user:pwd@DB-IP/sonarr-main"
When it completed the run, it outputs a kind of table that shows if there were any errors. In my case there were 2 tables (cant remember which ones anymore) that couldn't be inserted, so I edited those manually afterwards, so it matches the ones in the original DB.
It's pretty much a "develop from zero" situation. You can import assets, but will probably have to at least fix them up. If you are lucky, the two engines use the same language, but probably not. For example Unity uses C# while UE5 uses C++. And then you didn't even get to the parts where you actually use use the engine. Everything that touches the capabilities of the specific game engine need to be rewritten. That is off the top of my head: interaction, physics engine usage, collision engine usage, AI stuff etc.