OSX Timemachine backup never reaches its end point

I recently purchased a 6TB G-Drive to use as a desktop storage and backup drive

I've partitioned off 1.5 TB to back up my 500GB SSD and my 500GB External SSD using time machine

It's been plugged in for a few days now and its still not complete, it's backed up is way more than the amount of data than is on both drives combined (1.46 TB on the backup 322GB + 469GB on the ssd drives)

When I start up my computer, It says theres a small amount left to back up, mabye 150MB, it takes a minute or so to get there and then the amount it has left to do keeps growing, and it never reaches it

i.e.

when i start up it says 0MB of 150MB

once that's nearly complete the 150MB starts to grow

it says

150MB of 300MB...

1.5GB of1.7GB...

13.37 GB of 14.71GB...

It feels like its somehow stuck in a feedback loop and eating itself, although both the backup drive and the rest of the partitioned G-Drive are Excluded from the backup - It's only backing up the HD SSD + the External SSD

this just keeps going for hours until it throws up an error message saying something like 'there is a problem''

If i stop the backup it says 'waiting to complete first backup'

if i go into the drive in finder and take a look at the backup file it says (2019-04-07-090742.inProgress)

Im on high sierra 10.13.6

Thanks for any help