I'm using the FDroid version of K9. What do you like from FairEmail over K9?
But please bear in mind that my way of using email on my phone is very minimal. I usually just need something to read and for most of my productivity task I use mutt.
Do you mean just straight TLS connection at the start of session?
Few can handle authenticating to the server with a client certificate, but FairEmail seems to support it fine, although I've yet to configure it on my own server as it's likely to break other clients that don't support it.
Just to emphasise, this is probably not something that most people would ever need, but is certainly an important feature to me and something that FairEmail supports and apart from a rare few, nobody else does.