I’ve been using DeltaChat (available on F-Droid) for a few months now.
What I like about it is that because it’s email based, it uses OpenPGP for encryption, making it easy to have compatibility with other email-based solutions.
If you want to go the extra-secure route, you and your contacts can even self-host your emails - as long as you’re not going to send messages to people on Gmail or other big providers, you can avoid your messages being treated as spam.
The multi-device support is still a bit rough around the edges, but has gotten better in the last few months since the app is under active development.
Pretty much all the basics are covered, here are some examples:
And then of course all you power-ish user stuff (alternate launchers, clients for self-hosted clouds and stuff, terminal emulators…)
We’ve been using KitchenOwl (on f-droid) for a few months and we’re hapoy with it.