It took me hours of research to find the tools & documentation and haven't found any up-to-date text describing a similar process, so I decided to post my findings here in case someone else has the same requirement.
I wrote (in Delphi) a simple windows service that runs the FireSQL "nbackup.exe" command line tool to create a DB backup every [x] hours using this command line:
"c:\Program Files\Firebird\Firebird_3_0\nbackup.exe" -U SYSDBA -P [password] -B 0 "c:\path\database.FDB" "[output_file]"
The windows service keeps [x] number of backup files (erasing old files) and creates a new backup every [y] minutes. Personally, I used 1 full-db backup per hour, covering an entire week (168 files).
I then use the open-source SyncThing P2P file sync tool (also running as a windows service) to securely & automatically sync the DB backups with every PC assigned as off-site backup.
100% automated, secure, zero cost.