If you want 4k 120p - or anything with high throughput - the Z6iii is your option. The Z6iii is outstanding for video. It uses both CFExpress and SD cards. CFE allows high data throughput - a key enabler of 60p and 120p video as well as virtually endless burst length for stills. The partial stacked sensor provides a fast readout for AF, the EVF, and minimal rolling shutter. I worry a lot less than others about cropping a Z6iii image. What it loses in terms of pixels on subject it makes up for with less noise and larger photosites, so it is largely a wash if you have 1200 pixels or more on the cropped image. My rule of thumb for prints is I want 200 pixels per linear inch, but that's probably a little lower around 150 pixels per inch with the Z6iii.
The Z50ii is excellent for general video, has excellent subject detection AF, and has a high degree of capability for customization. The biggest disadvantage of the Z50ii is that it uses SD cards rather than CFExpress. That limits the write speed to 150 MB/s, and accordingly limits video at high frame rates without cropping. The use of SD cards also limits frame rate and burst length for stills. The sensor readout of the Z50ii is a little slow leading to a slightly delayed slideshow effect with silent shutter at higher frame rates. In practice I had no problem with the mechanical shutter for subjects with predictable flight - like large wading birds. It's much tougher for small songbirds where any delay could be problematic.
It's always a matter of tradeoffs. I own the Z8, Z6iii, and Z50ii. I have not found any of the cameras to have limitations that you can't work around. I can use any of these cameras to make excellent, professional quality photos.
As to lenses, the 180-600 is a clear choice if you can swing it. It's an excellent lens for birds. The 150-600 is good, but a little short - especially on a full frame body.
