Why is Time Machine backing up the entire disk every time?

Mount the Time Machine disk and check it in Disk Utility. It should state that the disk is formatted as APFS. If it's formatted with anything else. That is likely the culprit.

To fix, you either need another backup disk to start fresh or you erase and format the existing disk to be APFS then remount and make it a Time Machine drive. The Mac should start a full backup but subsequent backups should be taking APFS snapshots on the Mac and when the drive is attached, migrating the snapshots to the Time Machine disk and then 24 hours later removing the backed up snapshots.

Exclude ~/Library/CloudStorage from Time Machine backup. OneDrive, Dropbox, etc. create folders in that location and then use symbolic links to point Documents & Desktop to that location.

Settings > General > Time Machine > click the Options button and add ~/Library/CloudStorage to exclude it. Notice, I do the same with Virtual Machine because they will trigger a massive backup as well.

This should help with OneDrive touching files. Time Machine will ignore cloud drives like iCloud / OneDrive / Google Drive, etc.

Running multiple external USB HDDs can put undo stress on the Mac. If you need that many drives, I highly recommend that you consider a NAS instead. If you go with a NAS that uses the ZFS filesystem, even better. ZFS is superior to APFS, BTRFS, NTFS, etc. Once I setup a ZFS backed NAS and turned on the Time Capsule emulation. I stopped having failures with Time Machine backups. Time Machine creates an APFS sparse image which on the ZFS disk appears to be a folder with binary chunks called bands. ZFS ensures data integrity and as a result makes Time Machine backups more reliable.

Arq backup software has a very good reputation. Looks pricey but I haven't reviewed cloud backup pricing lately. https://www.arqbackup.com/

Are you using OneDrive or any syncing app that "touches" files when they are synced for that service? Any "touched" file will be added as an incremental backup with Time Machine.

+1 for @Mac Jim ID's recommendation for Carbon Copy Cloner (CCC). CCC has been around for decades from a great respected developer.

SuperDuper is another option and has been around for over a decade, but I found it has a higher learning curve as I only tried to use it once when I didn't have a valid license for CCC and I was in a rush to transfer someones data to another drive...actually found it faster to get my supervisor to purchase a new CCC license. My supervisor tends to prefer SuperDuper these days. Both developers & apps are highly regarded in the Mac community.

@James Brickley also has great advice.

It always comes down to your specific setup, workloads, needs, and amount of data involved.

Carbon Copy Cloner has been recommended in these support forums before, but I do not have any personal experience with it. Here is a similar discussion that may be helpful for you.

Is installing Carbon Copy Cloner on my iM… - Apple Community

same problem. APFS formatted. Every backup is the size of my entire HD data each and every hour.