Sony doesn't have aftermarket firmware but the firmware up to the A7ii has been hacked so you can run Android apps and control the camera that way. As far as photography is concerned there's no need for flashed firmware beyond that.

Have you got a link for more info on the hacked firmware? Hadn’t heard of it before. Thanks!

Oh wow, I had no idea this existed. Interesting, I have a RX100m3 and A6000 that can probably run it. On my RX10m4, they removed the apps feature, which I think was probably a good move for Sony despite restricting the feature set - they are really clunky to use (each app takes over the camera and has its own set of settings, takes ages to start up etc.) and the functionality they provide would make more sense either built into the firmware or as a phone app.

There are a few projects on Github wrapping the API which the PlayMemories mobile app uses (so you can take photos, access the viewfinder etc), which may be of interest to some: https://github.com/Bloodevil/sony_camera_api for example (I’ve not tried any of these)

Oh interesting, thanks for sharing that.