That feature req exists: Prompt to switch to encrypted chat when it is available
Perhaps you could file a feature request with K9?
If K9 implemented the SecureJoin standard it would be easy to share keys securely between K9 clients, and K9 would also interoperate very well with Deltachat; your testing has confirmed it. And there would be another excellent alternative to insecure key exchange.
Completely separately, from what you’ve reported, K9 could allow users to automatically create free e-mail accounts on any standard Chatmail mailserver, from inside K9, by typing in the e-mail address they want (or just picking the domain). That might be an appealing feature.
Alas, I can’t use K9, because I am on Mobian! So I can’t write up a step-by-step of configuring it. I take it enabling Autocrypt is pretty easy, though.
EDIT (because I technically can’t add a post)
It seems client interoperability has gotten more problematic: