Unless your willing to open a bug and be a pest for 2 weeks (and still have access to your old phone/leave it registered) I wouldn't plan on retaining your messages or keys across phones. Its a huge weak point of Signal.
Last time my phone died, I lost all of my Signal group memberships, because the app is incapable of transfering those to a new phone without backup... and it's also incapable of doing the backup automatically. Those UX choices continiously baffle me - it's like authors didn't learn anything from the failure of PGP.
It'd still be nice if the UX and reliability of restoring from backups could be improved.
Even if the backup works, the process is rather tricky. You have to get the backup file into a specific directory on the new phone BEFORE you open signal. There's no "import from backup option". If you mess up, you have to uninstall, then do the file copy.
There are issues like this going back years.
But it is a bit silly that you need to manually move the file from the old phone/backup location to the new one without some in-app option to do so.