Trash won't delete millions of Time Machine backup files

I moved some Time Machine backups to the trash, to save space on my external USB backup drives, and now they won't delete and can't be put back either. Now I have millions of files stuck in my trash.

I've been using Time Machine with my 2008 Mac Pro for years without a problem. I use a 2TB drive and a 3TB drive, so if one fails, I still have a backup. I started to get error messages saying Time Machine could not backup to the 2TB drive. So I checked it and it was almost full. I know that Time Machine is supposed to delete older backups, but I thought it might be struggling to do this. So I went into the backups folder and moved around 20 of the oldest dated backups to the trash. I also did the same thing on the 3TB drive, as I thought it would help. I've done it before, a few years ago, and, although it took ages, it worked. However, when I tried to empty trash this time, it came up with an error: The operation can't be completed because an unexpected error occurred (error code -8072). I have tried to select the files in the trash and "Delete immediately", but that doesn't work. It takes more than 48 hours to process the deleting and then it errors.

I have read online about using the Terminal to empty trash, but when I tried to do what it said, nothing happened. It's also a tad scary using Terminal, because I don't want to wipe my Mac accidentally. Is there anything else I can try?

The only thing I can think of doing is using Disk Utility to erase one of the drives and starting Time Machine backups again. Once that drive has a couple of backups on it, do the same with the other drive. It means starting again and I won't have any older backups to go to if I need them. But I don't know what else to do to delete the millions of files stuck in the trash. Any suggestions?