cross platform chat alternative to facebook messenger?

I am really struggling to replace facebook messenger / whatsapp for a few casual conversations. My friends and I are all wanting to move away. We are not heavy users of this but need it to work. I think the requirements are:

• floss client for android, linux, windows

• persistent history across devices

• reasonable security

• don't need to self host server

• can send a message to offline user, they get it when they come online

• not tied to or reliant on phone number / cell service

• ETA: end user documentation explaining how to set up and common troubleshooting

tried:

• matrix: the thing with having to keep track of room keys and stuff is too complicated. every time someone uses a new device it is a ton of issues and we could never quite get it ironed out

• signal: tied to phone number, no history across devices

• xmpp: similar to matrix the key situation is confusing, also no cross device history

• ETA: simpleX: a lot of people here are mentioning simpleX. It didn't come up in previous investigations so will give it a shot.

  • ETA 2: It doesn't seem to have persistent history across devices. Clarification?

I actually didn't think this would be such a problem but it is breaking us. we don't need a lot of sophisticated features like voice, video, moderation, 1000s of participants, spam protection etc that seem to be of concern to the projects. just simple text chat.