I think that was fixed a year or more ago. Just install new signal on new phone and when it launches it asks "migrate messages?". Click yes, the phone figure

I think that was fixed a year or more ago. Just install new signal on new phone and when it launches it asks "migrate messages?". Click yes, the phone figure it out, and you approve the transfer on your old phone. I've done it several times without problem. I had something like 40k messages, so it does take awhile, but has a helpful progress bar. About 2 years ago, it was much more painful involving a back and restore, which I got working, but was kinda painful.