Ive been looking for a while as well, and most iOS email apps are basically cloud apps where the app dev's servers log into your email for you, which I dont like.
Im currently testing Canary, which doesnt seem to do that and so far its ok. But I K9 and Fairemail are definitely one of few apps Im missing since migrating to iOS.
I’m using p=p on iOS - I saw it flagged as a K9 equivalent somewhere so tried it, and so far it’s good and table for me - let’s me see a lot of mails without the “external content” which at times , mostly, is just great
I’m not sure but I’m using Outlook on iOS and it’s been better than the regular mail app in my opinion. No idea about security and privacy though, after all I’m introducing another developer into my mails which can’t be too good I guess.