Is there a quick way to “flatten” an image in iOS
The best way I've found is to use Adobe Darkroom iOS app. You can view and edit photos from your regular albums, then export them to a flat file. Export to a new file, then delete the original via Darkroom. (I just checked, and realized I've been choosing the wrong option, 'modify original' - the details there say the image is still editable...damn). I just tested and in Darkroom I see markups made in the iOS photos app.

I wish apple would make a 'flatten' option! I'd love an app that flattens all edited photos before I export to windows.