Community extensions are exactly what I dislike about Flask. I want extensibility without custom extensions. I want the framework to be out of the way entirely when I choose additional packages to work with.
then don't use the extensions? Not trying to be snarky, I might be missing something, but does Flask actually make it harder to use additional packages?
Nope. You can use whatever packages you like. Those you've rolled yourself, standard python packages like psycopg2, redis-py etc., Flask community provided packages and anything in between.
I'm building a fairly large webapp in Flask and rarely, if at all, use any Flask packages other than the core itself.