What could be the cause? I/O completely died. Constant spinning, even logging in takes forever
Since this Mac is a backup, I would buy or find an external drive and install a clean OS there - leaving the internal drive and data untouched.

This lets you test the hardware with a clean OS with minimal risk to losing your data.

Then, since you don't have a drive big enough to back up all the data, you can wait to see if you want to throw out less important things until your data that needs to be backed up fits on the drive you have or you can fund a drive large enough to keep the data you choose to keep.

I wouldn't mess with things - slow IO and freezes out of the blue are usually two things:


fundamental errors / corruption to the data structures that a fsck can't or won't fix
hardware issues with spinning drives (SSD generally don't slow - they just fail when they can't accommodate any more bad blocks or the controller itself fails)


Both of the above, you need a backup to recover from in my experience and delaying making that backup, reduces what data you can save.