The key to your statement about the E-M1 is that you "use a grip now and then". The E-M1x doesn't give you that choice. Integrated grip bodies are big and heavy all the time, and that's a notable feature that's worth talking about, although it's not always a negative feature.
The Olympus PRO zooms are pretty good-sized lenses. An E-M1x plus an Olympus Pro 12-40 plus the Pro 7-14 is 253 grams heavier than a Z7 plus the Z 24-70 plus the upcoming 14-30 (and those are very comparable lenses - opinions will differ on which is "faster", since the Nikkors have a stop more DOF isolation, but the Oly lenses let in a stop more light)... Adding modest telephoto capability doesn't really change that - although there really aren't equivalent lenses. A Nikkor 70-300 G EG VR plus the FTZ adapter is almost exactly the same weight as the Olympus 40-150. The Olympus is almost certainly the better lens, although the Nikkor isn't bad. Alternatively, if you're willing to go without the zoom, the Nikkor 300mm PF on the FTZ is also just about the same weight, and is probably of comparable quality to the Olympus lens. I'd personally love to see Nikon make a Z-mount 70-300mm f4 PF.
Sony's A9 is very similar in size to the Z7, although some of the better lenses are heavier, with speed versus image quality tradeoffs that are closer to the philosophy of the E-M1x. While they're a reasonable cost/weight comparison, I can't imagine any image where a Z7 and an E-M1x are both the right tool for the job. You could concievably shoot landscape with an E-M1x or sports with a Z7, but why?
Of course, you can pick other lenses that make the weight difference huge - including anything longer than Nikon's 500mm PF and Canon's 400mm DO - or heavier options in modest focal length ranges.
Oly has a niche in relatively compact super high-speed cameras, and even the E-M1x is a great deal smaller and lighter than a D5 or 1Dx (but not a Sony A9, depending on lenses). An E-M1 mk III that brings some of these technologies to a smaller, less expensive camera could be a real hit.