Removing time machine files from external hard drive using terminal in macOS Big Sur

Everything in a Time Machine backup is related until Time Machine consolidates and removes that content on its own. Manually deleting Time Machine backup drive entries, by any means, is a really bad idea for backup continuity.

Prior to my upgrade to Big Sur, Time Machine used to organize its hierarchy under the initial Backup.backupd folder — by the name of the Mac. When I switched over to Big Sur, and formatted a new TM drive as APFS, what I see now is multiple top folders bearing the date/time of each backup, and within that, a Data folder (MacIntosh Data folder on a mechanical drive) containing Applications ... Users folders.

If you are using an APFS formatted TM drive, I don't know how you would difference other than by folder date, what is to be removed, and what is to be kept. If this were the old HFS+, then it may have retained the old Mac name separate from the new Mac name, and that would be simpler to trim.

TM will remove the old files as needed. Leave them alone.

If you really do not need your old TM backup I would erase the drive and start over fresh with Big Sur which would give you the additional space you desire..