Hi mate, thank you.
The source was the same MBP. I dragged and dropped all my Music and Picture files onto an external drive, and then dragged and dropped them to the same Mac after I reset it.
I'm not using iCloud for music or photos, only using iCloud for mail, calendar, notes and reminders.
Hindsight, I'm thinking what an absolute plonker I was. I've previously used the drag n drop method so many times without any issues, so thought it would work again. What I should've done was do my research first. Had I done so, I'd have realised I should've exported the XMLs for music and my playlists 🤦
I didn't have a spare USB drive, so I gave it one more go.... and I think it's worked.
No odd files in the bin. The storage data all looks as it should. The system data now is only 6gb, whereas previously it was showing at 300gb!
The only oddity was when I was transferring the Photos file. Even though there were no files whatsoever in the Pictures folder, when I dragged and dropped my pictures file in, it told me the file already existed. It asked if I wanted to duplicate it, to which I said yes. Once the file dropped in fully, there were 2 files, with my file being labelled with a "2". I then deleted the old file, and relabelled my one by removing the "2" from the file name.
🤞 all seems to be good so far.
One thing I want to ask for clarity as my brain is mush; when you import pictures from your phone, they drop into imported on the Photos app. If I delete the pictures in the Imported tab on Photos app, should they be deleted from the Library? Or should they remain?