For #4 I always tell people to assume 99% append only, but do consider the need for updates/deletes in edge cases.

If any of the data is PII and you will be subject to GDPR (you probably want to be at some point) then you will need a way to hard delete it.

A large portion of append-only datasets I’ve worked with have run into some edge case that required an update.

Also if you’re doing an append only log plus mutable view then please use triggers or materlialized views. I ran into a few cases where the approach was to just update both tables and that lead to divergences between the two.