Time Machine not backing up - stuck on "Preparing back up" for days

1. Disable/uninstall Norton and any similar 3rd party apps including dodgy performance monitoring/enhqncing apps. Do not ever re-install on this or other Macs - they are nothing but trouble.
2. I don’t recommend using Time Machine for setting up your new Mac because it will also migrate viruses/suspect software like virus cleaning that have accumulated over the years
3. Instead, manually copy data files like PDFs, docx, xlsx, and jpegs from your old iMac to external disk
4. Setup your new Mac afresh and after it’s done, manually install apps (avoid apps that bypass the App Store unless they’re from reputable, big-name companies), and copy over the data files from external

Good luck!

1. Refer to If Time Machine remains in the preparing stage on Mac - Apple Support.
2. Uninstall "Norton". Confirm its complete eradication, and then let Time Machine work. Never install "Norton", or anything like it, ever again. Rule 1 of Macs is don't install junk.
3. Then, migrate the contents of your new Mac from then Time Machine backup.

That's how I have configured all my Macs since the beginning.

Yes, but have you done it with a Mac over a decade old with hardware and software beyond obsolete? Setting up new afresh OTOH is an intelligent, fun exercise and a great way to learn one’s new machine. Plus it takes up very little time.

Time to move with the times, John, old ain’t gold anymore 😁

Yes.

Excerpted from Apple Support Communities Use Agreement - Apple Community,

"Test your answer. When answering a technical question, if possible, make sure your Submission works on your own Apple device before you post it."

When this is not possible or practicable I note that fact.

You’re welcome! Setting up a new Mac fresh is actually IMO part of enjoying a new purchase, so go ahead and enjoy! Any questions feel free to post, all the best.