Seriously, I would pay for being able to retain messages and conversations when changing to a new device (just through a simple restore of the device backup from the old one to the new one). That's the biggest selling point I can use with other non-tech people to get them to move to apps like Signal. Right now that's a huge drawback.
You can. On the Android app - Menu - Import / export. Allows to you to create a (non-encrypted) backup of your messages. I've used that when moving between phones.