A Z7II and the new 180-600 would run only a bit more than the original-when-new price of both the D500 and 200-500, maybe less when one considers inflation. The Z8 is still fairly new and is priced pretty high at around $4K, I get that.only a "no brainer/obsolete" if you can afford it,
Mirrorless systems are now mature enough and filled in enough to be worthy of taking the financial hit it takes to make the move. For frugal average hobby types, I get the hesitation. For those who have the means, desire and even the need, like always, the sky is the limit.
Honestly, trying out a kit via Lensrentals might be the best way to see how something might work for you. I have used them in the past for this and done well with the hands-on I gleaned.