Obvious suggestion, but: this is why I have my phone automatically copy new pictures to Google Pictures, OneDrive, and Dropbox as soon as my phone gets on a good wifi connection.
Maybe I'm naïve but I really cannot imagine Google shutting down Photos with no notice. So if they say they are closing it, I'll just downloaded them en masse and upload elsewhere.
May I ask which software/app you use that can do this?
I think all of those apps have an auto upload feature for your photos