The docs give an example of splitting pihole-FTL.db and state the individual backups can be easily merged when required. I didn't see an example merging them shown. Is it as simple as stopping pihole-FTL and then just catting them together? Like:
cp /etc/pihole/pihole-FTL.db /media/backup/pihole-FTL_latest.db
cat /media/backup/pihole-FTL_12-19.db /media/backup/pihole-FTL_latest.db > /etc/pihole/pihole-FTL.db