Identical files shouldn’t be re-transferred.
I am … let s say 95% sure, they won’t even be transferred on the first backup, and they surely won’t be on next backups.
In internet mode it will only transfer changed parts of the same file (you trade little a cpu for bandwith).
For storage, a full takes roughly the same additional space as an incremental.
For transfer, a full re-transfer and re-check everything, so you can do them not too often, like every 30-60days.
For internet backup and 15TB, you may search the forums for backup seeding. It works because the server won’t copy the same file even from a different client.
So you can copy the files to a drive, ship it to the server, register it as a different client. Make a backup of that client. Make a backup of the real server. Delete the temp client.
Also for 15TB, maybe that’s a little early/complex, but you can look at virtual clients to backup different folders of the drive at different frequencies.
Change Block Tracking won’t help for file backups, but for image backup it will speed things a lot.