A decent app would be okay as long as it is still compatible with the GoPro Hero 7 Black. I dread to think how long it would take to render 120fps 4k even on my rig, which is pretty good. I wouldn't complain about better image stabilisation either, but it is low light performance I am interested in.
I agree with the 8k comment. I'm only just now getting all my gear to work at 4k 60fps. I've never used the DJO Osmo Action so can't comment on that. I did buy a selfie stick for my GoPro so that when the new one comes out, I can use the GoPro Hero 7 Black to record my face.