Well a non-free app is obviously disqualified as an ideal solution. It's true that Threema has at least one advantage relative to Signal (not requiring a phone number), but I wouldn't want to use it mostly because of this review: https://soatok.blog/2021/11/05/threema-three-strikes-youre-out/

Also because there's at least one other messenger that shares that advantage over Signal, but can be used for free: Wire. I could be forgetting a downside of Wire though. Idk, I've looked into so many of these apps it's hard to remember the pros and cons of each of them.

I have been using it with some friends who refuse to use XMPP. Seems to work reasonably well and there is a third party client that seems to be tolerated on the network: OpenMittsu. OpenMittsu works, but isn't great to be honest.

Recently they open-sourced their client code (android and web, not sure about iOS), but uptake by 3rd party developers has been limited.

Bridging is also complicated by the fact that their API is for pay only, although a puppeteering gateway based on the OpenMittsu code would probably work.

Server side it is close-source and servers seem to be hosted in Germany I guess? Not 100% sure on the latter part.

Edit: The reason I insisted on Threema over Signal (only two choices they left me with cry) was that Threema does not require a phone number to work (but one can be optionally linked to an account).