For the primary lenses I use, I'd want the Z versions (the wide, and the 24-70ish). The converter might be nice to have access to the 70-300 until I get around to replacing it with a Z version. I have a few others, like a 50 mm 1.8, but it's not critical I have access to it on the new camera; I rarely use it on the D750. I'm also not above buying used, as long as they're in good condition.

I like the idea of a touchscreen but I want to try it out and see how it feels to use.

I know it's an investment, but at the same time, there's no point in throwing money away. It's a tax deduction, but that doesn't mean it's free; I still have to pay for it. (It just reduces my tax bill.) And there are some advantages to the Z6 over the Z8, like the articulating screen you mentioned. There are times when it'd be nice to be able to set it to a weird angle. (We're often crammed into a corner, trying to hide out of view of the camera...bathrooms, again, with their mirrors, are especially interesting. I often have to crouch down to avoid being "caught" in the frame.)

I did just spend a bunch of money on a drone, a 360 camera (for doing walkthroughs), and a phone stabilizer for doing videos, none of which I had before, so I already have spent money on higher priority things.

The D750 is obviously doing the job, so I don't really NEED a new camera, strictly speaking. That's why I'm not running out to buy one. But, if something happens to that camera or lens, I'm making $0 until I get it fixed or replaced. The camera is central to every job - without it, I won't get many, if any, jobs. If my drone (for example) fails, I'll lose a few opportunities, but I should still have photo jobs.