(A very tiny fwiw): you /can/ create a backup in signal and use it to transfer seamlessly to a new device, without triggering new safety number checks. The user flow sucks, but it is possible.
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.
In the interest of transparency I also use Keybase for things as well. The problem I ran into with Keybase my first go-round was I lost any ability to restore my account after I hadn't used it for about 9 months. That is mostly my fault for not having more bases covered, but had I had anything notable I would have wanted access to I would have lost it at that point. As an end user I don't feel that Keybase competes in the same manner as Signal. Since I'm an Android user Signal is 100% transparent to my normal use handling both SMS/MMS and Signal messaging. Keybase isn't a drop in replacement with that in mind. I realize this isn't the norm for iPhone users. Regardless, my question is: are many people using Keybase for messaging? About half my contact list uses Signal, yet I only have ever communicated with two people via Keybase.
For transferring backups from one phone to the other, I used Xender. Probably best to just transfer with an sd card, though.
It would be nice if this could be done on smartphone clients, but that pretty much requires leveraging some sort of "trusted key store" external to the app. (Which I'll admit may exist on some platforms, to varying levels of trust. How feasible and acceptable it is for this use case, I honestly do not know.)
on Windows: File, Preferences, Contacts - Import all signal groups and contacts from your mobile device. ... ... It is unclear what else gets imported.