At least taking your current apps (packages) selection (actually every single one, yet those that still use PureOS Amber should not expect that every single one will be accepted into PureOS Byzantium, not 100% but very close to what was installed before) with you should be relatively easy process (linked to my previous post, please note that starting with sudo su if executing any of those commands is necessary… although this might be wrong advice if related to usual Librem 5 setup, but for your purpose will do without myself rewriting them to include ), first step to be done by saving produced sudo.list file to some external digital data media. Restoring currently installed packages with xargs -a … would be second step, after brand new installation, as @Kyle_Rankin kindly already explained in his thread related post(s), including his related article: “Librem 5 Backup and Restore”:
P.S. Even if not helping much with my post, I’ve enjoyed using of remastersys package several times and therefore still remember that it was discontinued in April, 2013.