I do almost all my Google Photo editing on my phone. There is no way that I have found to create a custom filter. Occasionally I edit on my Chromebook or Windows machines (I dislike the interface compared to the mobile version). I have found a feature there, but I have not actually tested it. If you open an Edited photo, there is a 3 dot menu in the upper right corner of the image (see screenshot). Opening it gives you the option to copy the edits.
I assume you could create a set of edited images for different situations and then copy and paste the edits to new photos. I will have to test this out.
I wasn’t aware that you could create and save custom filters in Snapseed. Are you aware of the option to switch to another photo editing app within Google photo editing?
If you swipe to the left on the lower icons (crop, Adjust, filters, etc), at the very end you will find “More”. Tapping this will allow you to open any photo editor installed on your device. See screenshots below.
 I apologize for the images not being in the correct left to right order, but used G Photos collage in a hurry.
Maybe this would help? I never use Snapseed. Have you created custom filters in it?