pixel 1 offers full. The others offer regular. Either way its just an easy manager to find all of my photos. My main photos are backed up on my pc.
I am just looking for an app to streamline my backup process in real time as apposed to manually putting on my pc then onto the pixel to have my multiple backups.
There is
syncthing-fork and
Resilio Sync.
I personally use Resilio Sync as I found issues with syncthing-fork when disabling global discovery and it was chewing through battery.
But when Syncthing-fork works it's great.
With Resilio is you want a one way back up you need to setup the folder as a 'backup' / 'read-only' folder -
https://help.resilio.com/hc/en-us/articles/204762339-How-to-Back-up-data-Android-only this is essentially one-way sync, so if you make changes they won't come back down to your main device. Mainly deleting files on your OG Pixel once they been uploaded to Photos because of the limited space on them. Always good to use a test folder to get your head around the functionality of things.
My backup strategy using Resilio is this.
Main device ----> OG Pixel XL ----> Google Photos
||
||
||
Unraid server ----> rclone to Hetzner storage box & S3 glacier
Unfortunately don't have any pixel 1s in my phone collection 
It's a huge difference for me.
Anyway Google it, lots of people are doing it
Just have a look on eBay? I picked up a couple of cheap ones for this exact reason after Google changed the backup criteria.
Though they do run very hot some times, probably just need to replace the batteries.