Buy housing for my GX85, GoPro or other camera + housing?

The two posters before me made very good points, especially the cost of strobes, since you say you're primarily interested in stills. And if you go for a strobe setup with TTL, it would be wise to choose a camera with popup flash and a case that has transparent ports for fiberoptic cables.  If you want to dive or snorkel in 10 ft of water, then you may, luckily, be able to get away with ambient light.

But if/when you're ready to consider a different camera, you could study the camera reviews at DPReview - they favor the a6300 (gold) over the XT-20 (silver), for example - then make a decision for the type of shooting you do above and in the water.

Then I would go to a site like Meikon (or their sister site at Seafrogs), review what housings support your camera, and importantly what kinds of ports, domes, zoom gears, and vacuum systems (for leak checking) are available for that particular housing. Check to make sure ports are available for the particular lenses you intend to use - macro and wide angle. You would find that the a6300 mentioned above is well supported.

I have 3 Meikon/Seafrogs housings. They're cost effective for me, because I don't mind making fixes. On the Seafrogs case for the a6500, the rubber bushing on the mode dial kept rotating off, so I had to epoxy it on. On the Meikon Second Generation case for the a7Rii, I had to modify the on/off lever to get it to work reliably. Otherwise, all the cases work well and no leaks after about 50 dives.