TimeMachine "Backed up" but not? I'm so confused.

I've got a 5k iMac - late 2015, running Catalina. About 2 weeks ago everything became extremely slow (anything requiring disk access) I just turned on TimeMachine for the first time to get a full backup and I think there's an issue w/ my Fusion drive.

It's been backing up roughly 1TB of data to a synology NAS and it's taken a week. Now it's reached a point where the TM menu says "Backed up: 1.04TB" and the preference pan also says "Backed up: 1.04TB" but it's got a sliding gradient in a progress bar.

I'm backing up to a NAS and there is definitely that amount of data in there, however the NAS continues to show disk access.

I've run disk utility multiple times on all drives and it's telling me there's nothing wrong (although there are many warnings). However the slowdown has been excruciating.

Right now my main concern is making sure this backup that's happening is complete and accessible. Can anyone tell me what TimeMachine is doing right now with the gradient progress bar? Is it indexing things or continuing to add to the archive?

I'll also add that every 24-36 hours the computer has been rebooting. I'd like to make sure I have all my data, and then wipe it clean and start fresh.

I'm attaching images here of what I'm seeing. Thank you for any help you can give!