In addition to Polarr having a ridiculously good deal for the OS X app (https://photoapps.expert/forum/polarr-photos-extension-099-week-1999-nor…), their iOS app was updated over the weekend to include an extension for Photos for iOS extension too: https://itunes.apple.com/us/app/polarr-photo-editor/id988173374?mt=8
The app is free, but there’s a few Pro filters and features that can be unlocked for ~$20. I went ahead and unlocked it because I was so impressed with their their OS X app and want them to be around for a while.
Also, I found that I was unable to unlock the Pro features and filters from the app itself, tapping the buttons did nothing. What I had to do was enable the Photos extension, edit a photo and unlock it from there, which then threw me into the app and prompted me for my TouchID.