With Gmail (today, 30-May) forcing either OAuth or email app password (requiring 2FA across the entire google account just to get an email client working, no thanks) I have also switched to FairEmail.
For me the three step setup, which the first run of the app very clearly leads you through, was dead simple.
There are, as the OP mentions, many configuration options in FairEmail. Initially it doesn’t look like you have to delve into those very much, but it sure does have a lot of bells and whistles.
One note: with the Pro version of FairEmail it was a bit arcane to get per-account Notification sounds set up. Referencing an FAQ entry on this topic, those now work. Yes, Pro is required to have per-account Notifications. I was OK with the $6.49 one time price for Pro. I’m happy to support the developer this way.
Early days yet, so I don’t know how reliable it will be for me (instant notifs) but so far so good.
In particular, I eschew the Gmail android app because current incarnations [unless I’m missing something] don’t do instant notifications, i.e., IMAP PUSH. FairEmail, like k9, handles IMAP PUSH so instant notification of Gmail message arrival is available. (There are other things about the Gmail app I don’t like but we’ll leave those for another post.)
So the two main things I need: (1) OAuth to accomodate google’s password policy change happening today, and (2) instant notification (IMAP PUSH) seem to be working in FairEmail.
NB: FairEmail had a brief hiatus and left the Google Play store (removed by developer due to Google Play dorking him around) but the app is back in the Play store. At least it was when I installed from there earlier today.
Also, FairEmail is on github, so all the good interaction with devs usually provided there hopefully applies.