I hat to say it, but… Meta/Whatsapp is kind of the guy good when it comes to Big Tech messengers.
- cross-platform (including a web client that after initial linking to the phone works when the phone is off)
- E2EE by default
- still works on Android 5 (released end of 2014)
- can download the app from their website, no Play Store needed
- offers local backups without Google Drive
- notifications work without Play Services
I know, it’s not as good as Signal or Element or Threema or SimpleX by any means, but at least it’s much superior to what the other Big Tech companies offer, e.g. Apple/iMessage, Google/RCS, Microsoft/Teams.