Wouldn't be so sure about the 45 days for Signal. Had to do an "emergency" reinstall of Signal at one of my family members. All messages were lost.
However, Signal has a dedicated backup-function baked in that I've used before. (See under Settings > Chats) It should just do want you want.
It's called markdown. (More specifically CommonMark)
You can check out the supported syntax by Lemmy HERE. It has a lot of other features available to format text & stuff.