emptying trash on 2012 mac mini deleting still after 12 hours and currently at 850000 items deleted

Yes that explains the problem.

You should never manually delete Time Machine backups.

Now your TM backups are damaged and those backups are stuck in the iMac's trash.

One solution, is to eject and permanently disconnect the external backup drive.

The other solution, is to completely erase the external backup drive using Disk Utility and start over.

see > Erase and reformat a storage device in Disk Utility on Mac - Apple Support

It's now at 2,100,000 and counting.

That sounds like you dragged you entire drive contents to the Trash. 😱

I agree with hcsitas. I let the Mac handle it except for a roughly bi-yearly run-through of what is on my boot drive. Old, possibly unnecessary habits are hard for graybeards to break.

Before I empty Trash, I always peek. I do not empty it until I clearly know the items there are supposed to go. If such monitoring showed hundreds of thousands of files, that would set off a massive alarm between my ears! Currently there are eight items in my Trash and its not been emptied in over a month.

Ignore, it will finish when it finishes - could take days.

You’re welcome.

FWIW, I never empty nor peep - delegate it all to macOS, would recommend likewise. Emptying monthly/any schedule isn’t recommended either - you always lose most recent edits. Empty selectively when there’s a security concern, otherwise forget and enjoy!

It is still not clear exactly how or what ended up in your trash.

Unlike the others, I use a very different strategy for dealing with unwanted Trash.

I only put what I want to delete into the trash and then immediately empty the trash.