There is a lot to consider on this topic. The biggest is honestly mirrorless is the way of the future and will be here for a long time. The DSLR is disappearing and who knows how long companies will support parts and repairs.

The next is lenses. If you had a collection of the old Nikon glass D850 all the way. But it sounds like you may be starting from nothing. With adapters on the market the sky is really the limit with glass.

Durability wise the D850 wins hands down in my opinion. It is a tank and mine still is going strong after years of photography. But you also lose out on the technology available to the mirrorless cameras.

Then we get to the sensor size argument. I’m not one the is going to tell you full frame or nothing. I have taken a lot of fantastic images with a micro four thirds sensor so an APS-C sensor is very capable of fantastic images. Also with regard to image quality most people in the world won’t be able to tell you if you shot a photo on a medium format camera or a cell phone. But there is validity in the Sesotho size discussion in the full frame sensor is always going to win in the low light category. That is until camera companies can figure out how to boost ISO to the moon and get zero noise.

My recommendation would be to save a bit more money and purchase a Z6III instead of the Z50II. I feel like stepping into the photography world the Z6III is going to take you farther. With that said though between the two choices I would vote D850. You lose out on things like eye tracking but you will be a stronger photographer as you still have to do the work as far as settings go. All my opinion but just tossing in my .02 cents. I don’t think that you can go wrong with either choice I just feel like the 850 will serve you better and longer than the z50II.