Why can't I free up space on TimeMachine drive instead of wiping?

60 days is a hugely long time.

After about two weeks, Time Machine will do a full backup.

Just delete the entire drive and start over.

If you need to wait 60 days between backups, use a different backup solution. Time Machine isn't for you.

Carbon Copy Cloner...

http://www.bombich.com/software/ccc.html

Give Tri-Backup a try, it can do everything & far more than TM can do...

https://www.tri-edre.com/english/tribackup.html 

Your Time Machine backup disk is way too small.

TM keeps multiple copies and doesn’t re-save files it doesn’t need to; changes get patched into the file system within the Time Machine backup archive, and files that have not changed are not replicated. This to save storage. The oldest files are purged when that becomes necessary, but you’ll not want your only copy of the oldest file purged, for instance. So Time Machine doesn’t go strictly by date.

Time Machine does tend to get befuddled when mixing big changes with little available storage.

If you’d like to make a suggestion to the folks at Apple, here’s how: Product Feedback - Apple

Here, I’d get a four or six terabyte hard disk drive for Time Machine. More depth of backups, less churning.