Is something like this possible but in reverse? Not to migrate to WhatsApp, but to export data from Signal to a neutral environment (text file, zip file, database, whatever). I can’t switch to Signal until this becomes possible.
On desktop you can copy the appdata folder over to the new device. (warning: this is not officially supported)
Note that you must close the app on desktop from the host computer before copying the files over, otherwise the forward secrecy stuff breaks and you'll have to reset the sessions on the migrated computer.
FWIW it's nice to have a plain text backup anyway, and a lot better than nothing. Maybe it works fine but I'd rather not import a backup that has "weird" results.
[1] https://github.com/johanw666/Signal-Android
[2] https://community.signalusers.org/t/lets-talk-about-backups/...