Jule :mastodon: :tux: 🇪🇺, yes but open-source apps, the kind in f-droid, only get you this far. You would still need proprietary ones, that are only available on google play and depend on play services, to get many things done IRL. Yes, there are open-source google play clients. Yes, there's microg. It may work for something but would frustrate you endlessly in other cases.
Oh and also. Your custom ROM will most probably still come with proprietary drivers.