Thank you for elaborating this topics in a very god manner.
I have a been a little on the same route. Started with Canon 7d then oly mark ii, and now Nikon 500 as my main birding machine. I had the Oly mii with pl100-400, and changed for the oly 300 f4, which in my opinion was a better choice. Very sharp etc.. But now I am sitting with the d500 and a 500pf newly achieved. I still have the Oly gear, and my choice was: either go for the om-1 to complement my Oly 300 f4 or get the 500pf for my d500 (I also have the 300pf, prefer lightweight gear)
Why did I bought myself into ”older” gear and not the om-1? It was a hard decision, but something about picture quality was the turning point. I think the oly mii and now the om-1 are very fast and sharp together with oly 300f4. It is something with the oly thats too sharp.. . It is going surgical, almost painted. It is so sharp that it is looking a little bit unnaturally. That’s why I went for the 500pf, and I’m very pleased with it.
I don’t want to start a sensor discussion here, it might be, that I’m better to postprocess Nikon than Oly. And if there no Z6lll or similar dx substitute in the near future I might go for the om-1 also, because of its technical superiority.
And about the ”big white machine”, not for my budget

Thanks
/b