About FP and immutability specifically:

You don't have to go full on and re-write everything in Haskell. Small scale decisions can already make your life easier without much disruption. Eg if you are using Python, by default stick your information in frozen dataclasses, and only deviate from that with good reason.

Another thing I have a minor passion for is to as much as possible handle corner cases in the main line of the code.

Eg instead of having a big 'if' at the start of your function to handle an empty list and return early, try to make sure that the main logic can deal with empty lists just fine.

After all, the simpler your control flow, the easier it is to get code coverage in tests and production.

(I mention `and production', because in practice, lots of code is battle-hardened instead of sufficiently tested.)