Doesn't Signal already have backup? IIRC, when enabled it once per day saves all messages (encrypted with a backup key, which you have to write down somewhere) to /sdcard/Signal, and you can then use Syncthing or something similar to copy it to a new phone. If you put that /sdcard/Signal folder there before starting Signal for the first time, it'll ask to restore from that backup. WhatsApp has an identical local backup and restore flow (except that it gets the backup key from their servers, instead of requiring you to write it down).