Quicksy is the onboarding app we need for XMPP but until we have a perfect like for like Quicksy also available on iOS (like Element has) I can't in good conscience recommend my social circles join.

do you also use encryption with "regular folks" and verify each others devices?

That's the cool thing with Threema. If you verifyed the contact is so prominent, everybody want to verify each others key, even people who don't understand the concept. In Threema it is not hidden somewhere, it's just allways visible on each single contact with green or red dots.

Sounds similar to Element. However, Threema does not yet have the problem of having to deal with verification with multiple devices, (which is what I was aiming at)

YES !

first it automatically trust the devices but if you want you can improve it with manual verification :

https://gultsch.de/trust.html

"TLDR: Automatically trust all new devices of contacts that haven’t been verified before, and prompt for manual confirmation each time a verified contact adds a new device."