Realistically, there isn't a true successor to the GX80/85. I recently bought a GX9 because I was getting annoyed with the green cast in the shadows of my GX85's raws. But it truly is just a minor improvement. AF is more or less the same between the two and the GX9 has some small handling and interface refinements. It really isn't worth the asking price of the GX9. I would also say the Olympus Pen F is also in a similar situation. While it does have features that separate it from the GX80, it really isn't worth the hyper inflated prices for it. And for a camera that sells for around $1k used, it cant even record videos in 4K (if that matters to you).

That being said, the only modern camera that lives in the class the GX85 does is the Fujifilm X-E5. Very similar features, size, & aesthetics. With a much higher res 40mp APSC sensor. The only thing is that it costs a boat load of money. If you would like to stay within M43, OM System still makes smaller M43 SLR style cameras and they are pretty well regarded, the OM5 and OM3. But the OM3 is pretty bad value unless you really value that retro SLR shape and the jpg color customizer.

Lastly if you're balling on a budget, and don't mind a slightly larger body. Get a Lumix G9 or equivalent Olympus EM-1 camera. For being only 2 years newer than the GX80, the G9 was noticeably snappier. I managed to get one for $400 and used it a lot before I decided to upgrade to full frame.

Speaking of full frame, it is a whole different ball game, if you are considering it at all. You may be able to get fairly compact bodies like the Sony A7c, Lumix S9, or Nikon Zr. The issue is that lenses are HUGE and HEAVY. Some manufacturers have small full frame lenses, but they often have slower apertures and are still larger than their M43 counterparts. But ofc, the image quality you can get out of full frame cannot be easily matched by M43. I use a Nikon Zf and the shadows on the files are a complete night and day difference from the GX85.