How do I change the frequency of backups within Time Machine?

In addition to Bob's comments, here are three other methods you can try:

Method 1 - Via Finder

• Open Finder, and navigate to System/Library/LaunchDaemons
• Find com.apple.backup-auto.plist, and copy it to the Desktop (in case you want to return back to the default hourly backups)
• Open the plist file in TextEdit and find the StartInterval key. It has a default value of 3600. This value is in seconds.
• Change the value to your desired interval.
• Save the plist file.

Method 2 - Using Terminal

• Open a Terminal session
• Enter sudo defaults write /System/Library/LaunchDaemons/com.apple.backup-auto StartInterval -int 3600, and then, press <Enter> (Note: Change the value 3600 to the desired backup interval.)

Method 3 - Using third-party software

There are several available, here's one example: TimeMachineScheduler

You might want to take a look at Time Machine Editor.

https://tclementdev.com/timemachineeditor/