Time Machine not working with Big Sur

1. Ok - for those of you struggling to get time machine to work after a Big Sur update - This is what I've tried and what finally worked for me.
2. Once I had updated my 2015 macbook to Big Sur I re-formatted my external hard drive to APFS - I've always used either G-Drives and Seagate (cheaper) - annoying as I lost all my old bkps - thanks apple!
3. I got to about 880GB of bkps when my G-Drive started making loud noises and either got "preparing backup" or "Time Machine could not backup due to errors" - thanks for the detailed error Apple!
4. I ran disk utility on all my drives including the boot drive in recovery mode. I also left time machine running all night while it "Prepared the backup" - still no joy - at this point I'm tempted to re-install Big Sur - but I can't because I don't have a backup of my user account because my old Catalina backup is no longer compatible! Thanks Apple.
5. I deleted some old large files on my macbook to free up space for the snapshots - still no joy. (Shame as really I wanted to back them up first)
6. Uninstall CleanMyMac X and Sophos (basically any anti-virus stuff) - reboot try again - fail
7. So I bite the bullet and buy a new 6TB Seagate drive (I'll definitely have enough space now!) and configured it as APFS - ran the backup and got to the same point about 880GB then Time Machine starts timing out again.
8. I keep hitting the backup now button for the next 72 hours but still get the - oh so vague - "Time Machine could not backup due to errors" - warning! Thanks for the detail Apple.
9. I decided that either my macbook's hard drive is corrupt or my external drive is corrupt - so I exclude the external hard drive from the backup and by hitting the options button in the time machine preferences and excluding that drive.
10. I still get errors, so this time I exclude my main drive. Bingo! Approximately 10 days after I upgraded I get my first backup but only fro my external drive one. Whoop.
11. Then I read somewhere that some third party Apps not found in the App store can cause Time Machine to fail - So I then go into a process of excluding folders from my my drive on my mackbook, first the "Applications" folder, then my "Systems" folder and finally "User" folder. If I exclude my "User" folder - the backup works! - Great I have a backup of everything except my own stuff!
12. So then I got through and one by one exclude each "User", until I discover my own "User" folder is the one that makes the backup fail. Thanks Apple - surely Time Machine could have told me this!!
13. Then I have a brain wave and decide to try and copy my "User" folder over to my external drive and sure enough the copying fails and I get a very specific error, that says something like "There is a problem with your Photos Library and cannot be copied" - Bingo!!
14. After a quick search I find how to repair my photo library - https://support.apple.com/en-gb/HT204967 and after a couple of reboots I get my first full backup of my laptop!

So to conclude, one of the things to try if your Time Machine doesn't work on Big Sur, if you have space on an external drive - try copying the files over manually first - to see if any of them are corrupt.

It's ironic that it was apple's own software 'photos' was the one causing me all the trouble.

I hope this helps someone and thanks Apple! It's great when it works, but when it doesn't - Arrrrrrrggggghhhhh!!!