I've always believed that should should only buy expensive camera gear to satisfy a real need for your photography.
I've not always lived up to that ideal.
I upgraded from a D3400 to a Z5 last year. The major reason for me was to take advantage of the full sized sensor. That turned out, IMO, to be an extremely small increase in resolution. I have found that many of the other advantages of the camera have made it well worth the money I spent. I will have to say that right now the D850 looks to be a real bargain, especially when they put it on sale. Having said that, DSLRs are on the way out to be supplanted by mirrorless. Your first decision is do you want to stick with DSLR, or make the move to mirrorless.
It doesn't appear there will be any new DSLRs developed with newer technology, so if you want to keep going for longer with DSLR, a D850 would likely be a good bet for longevity vs price. If you think a move to the Z side is in your future, then perhaps saving your money for the time when making the switch is the right thing for you might be the best move.