Should I store user settings as JSONB or as real columns

Columns for anything you filter or join on, JSONB for the long tail nobody queries. The moment you write a WHERE settings->>'plan' = ... that runs often, that key has earned a column and an index.

timestamptz almost always. It stores a point in time in UTC and converts on the way out; timestamp stores a wall clock reading with no idea which wall it was on, which becomes unrecoverable once daylight saving moves.