Esteban

Get a focal reducer and you are now sharing the same mount
 
And a focal reducer can be got for only a hundred bucks, nothing at all really 
 

I mean you can get fast lenses for the GH5 without needing a speedbooster. Depends on what you are shooting of course. If you do lowlight stuff having a speedbooster can be nice. Keep in mind 1600 iso is iffy on a GH4 where as with a GH5 3200 iso looks really nice. Starts to take away from M43 though when you throw a speedbooster with heavy glass on. Might as well just have a full frame camera. 

In terms of color matching HLG on both cameras is probably the closest thing or CineD. It will be different though and at that point it doesn't really matter what camera you get. 

That's a valid point, too.

Personally I love using m43 cams with native lenses for interviews. You can easily light for f/2.8, but with the benefit of f/5.6 DoF.  It means I can use smaller, less powerful lights which helps keep the setup small. 

I would save for another S1 honestly.
Even the jump in terms of color science and dynamic range between the GH5 and S1 is quite remarkable.

If you can wait a little and don't need a camera right now, the XT4 will be announced at the end of the month and shortly you will be able to find used XT3 quite cheap

yes

Only available in virtual reality?