delete time machine backup

I'm running a new M1 Mac mini. Big Sur.

I have an external drive that I'm using for Time Machine backups. The problem is that there are backups on that drive associated with a computer that I no longer use, and I would like to remove them to free up the space.

I tried just dragging the folder associated with that Mac to the trash and emptying the trash. I did that over 24 hours ago. I'm currently at 630,000 items deleted and there's no indication of how long this is going to take to finish. Do I have any options other than just waiting for it to finish?

(I suppose I could cancel the deletion, wipe the drive, and start over - but there are time machine backups there which I would like to keep if possible.)

I no longer have access to the old computer which is associated with those backups.