bendud: I (clearly!) don’t understand the problem here.
bendud: So IDK how your data didn’t move across (it does take time).
@bendud, the OP's phone has a file system coruption or similar. It's broken, faulty, failed, not working, kaput. The data can't be correctly read from the phone.
It's right there in the first post. The phone is a 256 GB device reporting 81 GB used, yet the backup was still going at 300 GB. A backup size of around 81 GB would be expected. The category of the data, be it other/photos/whatever isn't relevant to the tethered backup.
bendud: Do we know that though? OP hasn’t said how much storage their iPhone had and the process of backing up does need more space than the phones storage might suggest, and those “other” categories of storage are famously inaccurate.
Agreed and I would stay away from iTunes 😅
My first option is always iCloud restore. I would also recommend the direct transfer from iPhone to iPhone but you will need to wait for the transfer to complete on both devices before you can use them.