Hello, Adam - I'm glad you enjoyed reading it!
Comparing the Zf and the OM-1 is a bit of an apples-to-oranges situation, or maybe more like apples to Volkswagens :-)
Since you have a Z6, you already have a good idea of what the sensor in the Zf can deliver. The new processor makes it even more performant, especially at high ISOs, so there are no mysteries there. You also have a Zfc, so you know how Nikon's approach to the physical dials works and, importantly, how it compares to using the Z6, which operates much more like most other digital cameras (including the OM-1 in that regard). And you have some Z lenses, so no need to sell you on the quality of the glass or the ecosystem.
If you like the way the dials work on the Zfc and would prefer to have a full-frame version of that experience - plus better build quality and nicer-feeling controls - I think you'll really enjoy the Zf. I had planned to sell my Z6 to help fund the Zf but decided to keep it because its control layout and ergonomics allow me to work faster and without taking the camera away from my eye.
The OM-1 is a whole different animal from any of the Nikons. In a sense, it is the "anti-Zf." OM System did a wonderful job re-doing the menus in the OM-1; they are much more logical and intuitive than the previous Olympus menus, which were, frankly, abysmal, and that's saying something :-) That said, the OM-1 is so full-featured that figuring out how to set it up and make it work for your shooting style requires a commitment. Once it's set, it's as easy to use as any other camera, maybe easier.
It has a combination of features that you just can't find elsewhere all in one place, including live handheld ND, live composite, in-camera focus stacking, pro capture, and incredible image stabilization. Yes, other cameras offer one or more such features, but not all in one camera, as far as I know.
There are other more obvious differences between the OM-1 and any Nikon Z, including sensor size. Both have advantages and drawbacks, and I'll assume you're well aware of those, though I will say I am highly impressed with the image quality, sharpness, and noise control from the JPGs right out of the OM-1. They're impressive.
I got the OM-1 primarily for wildlife and birding because I couldn't manage the size, weight, and cost of the full-frame telephoto lenses. m4/3 offers a huge advantage on the long end of things.
As far as the lenses go, I would put the Olympus PRO glass - particularly the zooms - right up there with Z glass, at least to my eye. Fast-focusing, super sharp, and contrasty, and they're a joy to use.
The OM-1 ended up being the basis for my primary camera *system.* And although the Zf could certainly do the same, I regard it as more of a specialized camera due to its controls. When I'm in the mood to slow down, take my time with physical dials, and simply enjoy a really well-designed and well-built camera, the Zf is a real joy. When I want to go out with a highly capable "one body does it all" camera and *system* in a single bag and not have to think about the camera as much, I grab the OM-1.
So, back to your choices! Since you have the Z6 and the Zfc, there is a certain "more of the same" quality to the Zf due to the overlap between it and your two current bodies. That isn't necessarily a bad thing, especially since you've already invested in some Z glass. The OM-1 is completely different. On the plus side, it has a lot of advantages as a body and as a system, and it can do things few other cameras can. You'll get a smaller sensor, but only you know if that truly matters. The weight and size savings - especially on the telephoto end - are significant. You'll be buying into a whole new ecosystem which can be expensive, but you'll be getting something completely different.
All of that, and I'll bet you've no more clarity about the decision than before you asked! :-) I would love to know what you decide on, though, if you don't mind sharing once you've made your choice?
