The D7500 is a very capable camera. Shouldn't really limit your photography (based on your stated areas of interest). Personally, I don't think it's a good idea to switch to mirrorless just because everyone else is doing it. You should have a valid reason for the switch. Something like "if my camera could focus on eyes automatically, my portraits would be better". For me it was "I sure would like to use my old Minolta lenses again".
By last summer, I had been reading about the new Nikon mirrorless cameras and had become interested in seeing what the hype was all about. Much like where you are now. And, as I said above, those old manual Minolta lenses that I had saved for 40 years were sitting there on the shelf enticing me to use them. I had switched to Nikon back around 1990 and owned a D40 and a D7000 with quite a few lenses. When the Z5 went on sale for $1000, I decided that I could handle that amount of money to find out what the Z cameras were all about. I got the camera with the FTZ adapter which allowed me to buy the camera body only and use my old Nikon lenses on it. I also got a KF Minolta MD-to-Nikon Z adapter.
Two things were immediately obvious to me when I started to use my Z5. First of all, the auto focus does not work like it does on my D7000 (and your D7500). I won't go into the differences here (you can find MANY discussions on the subject here on DP Review) but figure on spending some time learning how to get the best performance when using AF. Secondly, I was somewhat blown away by the image quality. This was my first FF camera and despite never have been disappointed by the image quality of my D7000, I was amazed at the quality of my pictures, even at what to me were very elevated ISO settings. Your D7500 is sure to be better than my D7000 so the difference won't be a startling, but I think you'll be very happy with how your pictures look (once you get the focus business worked out).
If you have any interest in using old manual focus lenses, it's a no brainer. Get a Z camera right now. I have had more fun with my old Minolta MD lenses than I've had in photography for a long time. I had forgotten how much fun it is to put the plane of focus just where you want it using a 50mm/f1.4 lens. With the focusing aids on the Z5, focus is much easier than it ever was on my Minolta cameras.
I know you were thinking of getting a Z6 (the Z6ii I assume) and I can't fault you on that. However, you might be able to get a Z5 plus the 24-200mm lens for around the $2000 you are willing to spend. The Z5 doesn't give much up to the Z6 in features (image quality is virtually identical) and the 24-200mm lens is an outstanding walk around lens, adding a very useful couple of mm's on the wide end compared to your 18-140mm. The z6 and Z7 are likely to receive an update fairly soon, picking up some of the advances brought by the Z9 and when that happens, you could add a more capable mirrorless camera to your Z5.
Since you only have the 18-140mm lens, I'm not sure how much sense the FTZ makes for you if you end up getting a Z5 with the 24-200mm. If you owned other lenses, getting a FTZ seems like a very good idea. Just not sure it would be worth it to you. If you by a Z6 body, you'd need either the FTZ so you can use your 18-140 in DX mode (a not very attractive proposition in my opinion) or a lens to go with the camera. The logical, budget choice would be the 24-70mm/f4. You could probably pick up a used one pretty cheap. By all accounts it's a very good lens. Personally, I've never cared much for the 24-70mm range. 70mm always seemed too short on the long end. But a lot of photographers love the 24-70mm zoom. In your situation, getting a Z50 and using the 18-140 might actually be the best way to get into the Z cameras (you'd still need that FTZ).
Not sure my comments have been very useful. The takeaway should be that if you are getting a Z camera, you should do it because it will enable you to take better pictures or that it will allow you to enjoy photography more (as it did for me).
Bruno
