I'm curious if you've looked into Signal and if you think it's a viable alternative to Threema? Specifically considering Signal is fully open source (client and servers), uses reproducible builds which allows you to verify that the source matches the app you're running, and is owned by a non-profit which can't be sold to a for-profit company.

'pay once, chat forever'

I don't see how thats tenable with anything that requires a hosted server to relay information.

Someone will need to pay for it going forward and if the users money runs out, what then?

spelling mistakes on the landing page