Sorry nobody with more intimate knowledge of the gear in question has responded here.
My take on it is that you should ask yourself what, if anything, your current setup is preventing you from doing, and whether the proposed upgrade will make it so.
There's a pixel count difference between an Z6 and Z7. If you go with a Z6 you get no more pixels than your current rig, but with a Z7 you do. This is of no consequence if you don't do pretty extensive cropping, but if you're cropping a lot, the Z7 might be an improvement.
Obviously there are some big differences between mirrorless and SLR, but you pay a lot for them, so you have to decide if you need them. I think the mirrorless could have an advantage in size and weight, and if you're interested in certain older non-nikon lenses, the availability of adapters is nice. You get an informative viewing screen. But the SLR has a few advantages as well, including longer battery life, better protection against sensor dust, and an abundance of relatively inexpensive lenses, including a good bit more available at the long end. If you intend to continue using the lenses you now have, you'll need an adapter, and with it some of the advantage of the compact mirrorless design will be undone.
I don't know how focus speed and accuracy compare, but again, I'd base some judgment on whether your current rig has any shortcomings there that have hampered you.
In the end, though, it's also partly a question of budget and love of gadgetry. It makes a difference if you have plenty of money and a love of new stuff. Some people get all up in your face for wanting the latest and greatest for what they consider inadequate reasons, but really in the end it's your judgment of reasons that counts.
