So, I have a Threema license, but from what I’ve seen its encryption isn’t post-quantum. Signal’s encryption seems the strongest. I host my own matrix server.
Also, I kind of don’t care where the servers are or which provider it is. Everything is encrypted anyway.
Post-quantum isn’t really a big problem because it will be a very long time before there are viable quantum computers (maybe never). You should focus on the very real risks of security breaks from normal negligence and design errors.
Threema seems pretty unpopular, so the risk is highest. Signal and Matrix are both popular and have a lot of scrutiny on their cryptography.
All 3 have open source clients, but Signal contains some binary blobs. Only Matrix has an open source server, though end-to-end encryption enforced by the client alleviates most of the concern of proprietary servers. All 3 support end-to-end encryption.
Threema has been audited multiple times.