While this (Threema) is Open Source, I feel it does not align with the spirit of FOSS.

See the commits here for example: https://github.com/threema-ch/threema-android/commits/main

It's one commit per version. Contributions go to a private email instead of a public mailing list. All development happens in private and pushed to the public in a version bump with no changelog.

So yeah it has an AGPL License (which is good) and meets the minimum of FOSS requirements, but it gives of the feeling that they're following the letter, but not the spirit of FOSS.

I would definitely not use it or push any friend to use it.