The nikon adapter is just a tube for spacing and a few contacts.
I went mirrorless a while back. Love it, still shooting a Z7ii, but the best part about the Zs is using the Zs native lenses, which outperform what previously I thought were almost perfect lenses. Like the 20mm Z is better in the corners than the 20mm G lens I absolutely loved. The vibration reduction is both in the lens AND on the sensor and I can genuinely get an extra stop or two over my previous digital with VR. It is almost trouble free to do crap like hold it over the heads of a crowd and spray and pray. The Z kit is just fantastic for my sorts of digital uses. And the sensors they're using have unreal dynamic range, so you can dig a lot out of the shadows of the latest Nikons.
But mirrorless has drawbacks. Small camera, but the lenses are very large for what they are. And cost more than legacy lenses. And battery life is significantly less than a DSLR with the same sensor. But for being stupid big and spendy, the optical quality is just spectacular.
Now, if you're going to use your current crop of lenses exclusively, DSLR is fine. The FTZ adapter works for anything that has the focusing motor built in, but your AF-D won't autofocus on it. I still use some AF-D on my Z7 and hand focus for a lot of stuff, but it's stuff I would hand focus on anyway and the digital viewfinder actually helps me there -- focus peaking and a hand focused 105 are a fantastic rig for shooting musicians on stage where the autofocus will try to grab the mic, not the musician's face, for instance. I can make sure the eyes are in and also see the real DOF even in weird light and cluttered environs, which is a game changer for me. But you'll still see the disadvantages of the mirrorless and your image quality won't be better.
The latest gen of DSLR using the same senors will have the same IQ with your older lenses. And they'll autofocus on the ones with the screw drive, where the Z7 with the FTZ will not. Likewise, you don't have the disadvantage of the Zs, like the shorter battery life, which is significantly shorter. I always have spares because the non-optical viewfinder uses a bunch of battery, just like using a DSLR while focusing with the back screen, so I burn through them. Expect about 3x as many shots on the same battery using DSLR as on Mirrorless. Also, some other small things bug me about the Z7, like how I access exposure modes with the i button, or digging for settings in the Menus, but my last two DSLRs also had quirks, so I don't know if an 850 would not have something that bugged you. Nikon's menus and software functions are... well, Nikon has never been good at software. They're not getting better. We'll leave it at that.
TL: DR I'd get a DSLR if you're only ever going to use your legacy lenses. I'd get a Z6ii or Z7ii if you were also interested in the new Mirrorless lenses, which are absolutely phenomenal.
