I think having something that’s the best of both worlds would be the better approach. Sprite Factory stands alone by itself, not necessarily tied to MonoGame.Extended or even MonoGame in general. I would just ensure that the exported JSON is documented well enough, and has enough properties, that it’s easily digestible into whatever framework/engine a user chooses. Then having an importer written for MonoGame.Extended would be just one example of how to use the tool.
With someone writing an importer for Phaser 3, that’s another environment that would support the tool, and for something more vanilla MonoGame, I wouldn’t mind putting something together for that. When i wrote the importer for Aseprite animations, I tried to keep it generic enough that I could work easily into any Monogame project and could piggy back off that for this importer.