I ended up with an OMDS OM-1 and 300mm f4 as I got them both new for a great price (almost 1000$ less than a Z8).
Got a good upgrade in both camera body (black-out free EVF, 25 fps, animal detection AF) and in lens (faster AF, brighter lens, sharper image quality). This allowed me to get keepers in the past months that I wouldn't have gotten if I stuck with my original choices.
As such, I think that if you get hung up on either the Z9 or 500mm f4 E you are making a mistake.
If you get the Z9, you'll still be limited with the 200-500mm f5.6, which, though not a bad lens by any means, it is slow to grab initial focus, has average performance at longer distance and has quite a bit of focus breathing (I'd say it's a ~450mm when fully zoomed and focused under 20m).
If you get the 500mm F4 E, you'll get a stunning lens but you'll be missing out on all the bells and whistles that high end Z mirrorless brings and that allow you to get more keepers and make shots easier. You'll also need to adapt to monopod/tripod with gimbal shooting...
I think the smartest thing you can do is this:
Get a Z8 and FTZ now as it's got a great price these days.
Put the 1500$ or so you saved over getting the Z9 back in the savings process.
March or April 2024 sell the D850 and 200-500mm f5.6 and with what you saved above and in the mean-time get a Nikon 400mm f4.5 and the Z 1.4x TC.
P.S: but if it must be a Z9 or 500mm f4E, then get the Z9 and save for the 500mm f4E as by the time you have the money for it, you might find better deals for Z-mount telephotos.