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

Welcome to the world of expensive choices.

Consider carefully what you want to do while diving, what you want to do with the resulting images, and if you want mostly video.

I'd suggest that a GoPro is the best starting point for a beginner concentrating on video.

For stills, the choices you list seem fine to me. But the suggestions that you will need more equipment are also correct. But there is no reason you can't start with ambient shooting (especially at less than 30 feet) and at least get used to shooting while diving and get used to the post-processing necessary for underwater work. (In that respect, make sure any camera you buy will shooting in RAW mode. You'll need it.)

TG5 is a nice choice too, based on the images I've seen. Myself, I've used a number of Canon point-n-shoots (no longer made) in matching Canon housings over the years. All cost about $400-500 new for both camera and housing. My s120 is the latest iteration.

I also shot a Sony RX100 II in a Nauticam housing for a year. NIcer output, but the menus and ergonomics are quite a bit worse than any of the Canons. And at least with that model, the autofocus was just as slow as the point-n-shoots. Ultimately too slow for me.

Suggestions for a new diver:

-Get a camera without external flash that can be dangled from a wrist strap on the right wrist. (Cameras are made for right-handed shooting, generally).

-Always put the wrist strap on as the first thing you do, either before going into the water, or as soon as you take it out of a pocket, have it handed to you from the boat, etc. I've seen a diver lose his new Sea & Sea $1000 rig as he got on the boat, thinking it was strapped to him. He didn't have a routine with it yet, after a few days on a liveaboard with it. He let go of the rig to get onto the boat and it floated away. Worse, he thought it would sink so the whole boat went on a search grip operation looking for it on the bottom while it (likely) floated away. He had checked bouyancy in a fresh water sink before the trip, where it sank instead of floating.

-Do not add external flash or a handle until you are familiar with both diving and the camera. Adding a handle tends to me you are gripping it the entire dive, No dangling from the wrist in times of stress. And hanging onto a camera by handles tends to change how you dive, makes you pay more attention to the camera. Adding external strobes makes it more unwieldy, adds significant drag besides the expense.

-Get a camera with the fastest autofocus system you can afford. It would be good if it had a spot autofocus feature and a joystick to move the focus point.

-Touch screens will be of no use to you underwater.

FInally, if you want to get started fairly cheaply, a universal smart phone housing might be just the ticket: https://www.housingcamera.com/id4850-itorch-krh02-housing-4-universal-smartphones.html