My preferred solution would be phone -> local backup (for safety) -> Google Photos (for convenience). If the sync to local is about as good as google this would be the best for me.
Distant Plan b: phone -> google photos -> local backup. The problem here is that the only way to get your originals out of google photos is to use google takeout which is basically a dealbreaker here.
My solution is same as your plan A. Except that local backup is a NAS that also stores all backups from computers as well as photos from phones. Then everything goes to back blaze and photos go to Google photos for convenience of sharing.
