"Signal is licensed under the AGPLv3 and as such, Free and Open Source."

Correct. Unfortunately it's also rigidly centralised. So state-level actors only need to block one set of servers and it's unusable (eg Signal is very hard to use in China due to the Great Firewall). Matrix is a better bet, both E2EE and Free and Open Source software, *and* a decentralised protocol anyone can implement.