You already have very good cameras. If you can not decide between several other cameras then wait. In time you will come to a conclusion and then buy it when it becomes clear. If you just can not wait go to a store and try out the ones you are considering. Remember there might be a new Panasonic released later this year maybe even an new Olympus.
That is sound advice , I have had the GX8 since it came out in 2015 and it is one of my most loved cameras ever. I do miss not having decent IBIS for video. The pixel shift thing is more of a would be nice rather than a must have. Plus no matter how long I wait the GX8 true update will be announced the day after I buy the new camera

I am just finishing my tea before getting back to fence building so plenty time to think it through. But your advice is sage
Late to the party, but my 2 cents....
I agree with DLBlack. Wait.
Here's my little(?) recent revelation:
Bought an RP and a few adapted lenses a year ago. Was going to keep the GX8 and a couple of lenses (100-300mm, mostly) for long tele, and definitely keep the GM5 and kit lens for portability. Sold off all my other M43 lenses, and, after thinking about consolidating even further, decided to try the Canon M50, which would enable me to use my adapted EF 70-300mm (not L) and cover almost the same focal range as the Panasonic 100-300mm. Doing that would keep down the amount of gear to carry, and keep the lens options open for both RP and M.
So I did it. Got an M50, and a few native lenses too. BUT. There are a couple of nice native lenses in the M line, but the M50 (and definitely not earlier models) S-AF is not as fast or precise as that on the GX8. There is NO native normal range tele that is useable; forget about actually good. They have a great wide zoom, which almost makes up for that, but... And no native long tele (but I knew that going in). The sensor is quite good, though.
I kept meaning to put the GX8 up for sale, but never quite got around to it. So, when flower season started this year, I decided to pull out the GX8 and see how it stacked up against the M50 (and an M6 I had acquired also on a wicked Black Friday fire sale

)
Well, you know. After arguing with the M50 (I actually like the M6 better, but it's painfully slow to focus, so...), and getting some decent shots, I popped the GX8 and 100-300mm out of its box, where I had stowed it prior to selling it, and stuck on an extension tube, and shot away.
When I downloaded the M50 shots, there were some nice ones, and lovely color gradients in them. But, they were very...average. No grabbers. The GX8 shots? Damn, some shots just jumped right off the screen at me. Okay dokay....sigh....
So, after about 8 months with the M system, I decided to box the whole kit and kaboodle up and put 'em up for sale. For now the RP stays, as FF does do different things than M43, though.
Now, what does that have to do with your dilemma?
I really like both the GM5 and GX8, still. But, none, zero, of the current Panny bodies does anything for me that would inspire me to "upgrade". And, from experience, I find the Oly UI less than ideal for me, so I have no interest at all in their bodies. Anything else Panasonic has out there is an incremental change from the GX8, with some better and some worse attributes, none of which matter to me enough for me to take the plunge. Of course, I also do not do video, so since that is an issue for you, your decision might be more complicated than mine...but...
What would make me upgrade my GX8? A genuinely more advanced sensor, in a body that's not bigger than the GX8, and an EVF on par with that in the GX8. Added to that, of course, the low-shock mechanical shutter and better IBIS, and I would be all in. However, until I see that camera in the flesh, if it ever appears, I honestly don't think I will be updating the GX8.
So, given that, and also given how much you like your GX8, I would say.....WAIT. Unless you have a camera failure, or desperately need the video upgrade, just wait.