Difficult question ... The only one who can make that choice is no one else but you.

Yes Z7 is a phenomenal camera, but in the mirrorless I would rather choose for Sony I think. But that's a personal choice. I don't see the DSLR disappearing soon ... The mirrorless is still very new and very expensive, for now I think there will be new DSLR's coming too.

I have the D750 too and I think it's a phantastic camera and perfectly capable for shooting what you want. Don't know whether you are pro and make money with it, or hobby and only give money to photography.

If you are hobby photographer, I wouldn't make such an expensive switch right now ... In that case I would settle for one new lens so you can expand the hobby to what you want to do.

But that's me, and probably I do have to worry a lot more about the money. Wish you good luck with this choice.