can't backup

My time machine doesn't back up as it should. I've been trying to figure it out and get support for this for month.

Normally it backs up to an NAS. I realized that it hadn't actually properly backed up in a few weeks, so I tried to get it to back up then and it said "backed up: 5.91 GB" which was confusing. I rebooted my NAS and laptop and then got it to do a proper back up to the NAS. 

But since then it hasn't been backing up properly. It says "backed up: 1.05GB" and then "backed up: 1.65GB", "backed up: 3.05GB" and so on. But I'm not sure where it's backing up to, but it's not the NAS. 

I'm not having luck finding information on this behavior on the internet, just one support thread that was "resolved" but with no answer as to HOW it was resolved.

I'm running Catalina (10.15.7). 

I've done the following to troubleshoot:

Backed up to an external HD. This functioned fine.

Reboot NAS and laptop, clicked on back up. This worked before, but this time I got "backed up: 2.15GB" or something like that. 

Verify back ups: got an error that they couldn't be verified because it was interrupted and this may be due to a network issue. So I decided to look more closely at both my network (router) and NAS. 

I connected to the NAS drive's dashboard to check settings and run diagnostics. Came back fine. 

Transferred files to and from the NAS via the Finder window with no issue. 

Checked settings on the router and rebooted both router and NAS. 

While those were resetting, I unmounted the hard drive from time machine and rebooted my computer. When everything was back up and running, I tried to back up again. Still doesn't do anything other than increase the size of "backed up: X.XX GB".

Verify back ups again -> no error messages, task seems to have completed successfully

I tried to back it up again but now the message is up to "backed up: 5.05 GB" and that is increasing until I stop the back up.

Router appears to be fine as all traffic directed in and through it is fine. NAS seems to be operating normally on its own in terms of data transfer and being recognized on the network. The time machine program seems to be functional. However, the combination of NAS + router + time machine program is NOT working together and I can't figure out where the kink in the system is or how to get it fixed.

Since then I've also done the following -

Add a second NAS to the network. Backing up worked fine for about a month and then the same issue happened.

Deleted original back ups and started a new one from scratch. Took about 2 days to make a new backup from scratch but once it did, it backed up fine for a month before starting the same issue.