The GH5 is basically a video camera that can take stills, the A7III is an excellent all rounder, it just really depends on what kind of codecs you need and if you need 4K60. If you want the best middle ground look into a used X-T3 or X-H1, look at the lenses you need/want and budget accordingly. The GH5 is a great video camera, but I've just never been too interested in M43 with the amount of indoor work that I do. What would you be photographing/video recording?
I do mostly fashion and product photography, and for video I'd be looking for long-form recording if possible. I'll look into the X-T3 and X-H1, thanks!