While the camera itself is important, you also should look beyond it to the rest of the system to be sure what you get will meet your needs. For example and based upon your expressed interested, does the camera manufacturer offer portrait specific lenses or architectural lenses that best meet your needs?

The Nikon D850 is one heck of a camera. But is it "more than you need"? 46MP image files will be big... filling up hard drives and memory cards pretty quickly. You would need to make huge prints to make full use of such large image files. Would 30MP be sufficient? Or do you need even more... 50MP?

Does the Nikkor lens lineup fully meet your needs and is it the best value? Depending upon the type of portraits you shoot, you might want f/1.2 lenses, in which case the Canon system might be a better choice since it includes a number of those. Both Canon and Nikon systems include some architectural lenses.... Canon tilt-shift or Nikon perspective control lenses. But, compare the available focal lengths and prices. A starting point might be to compile a list of what you feel your current Pentax kit doesn't provide for your work, then look at the other systems to see if they fulfill those needs.

No system is "perfect". But the Nikon and Canon systems each have about 60 or 70 full frame capable lenses to choose among (far more than other manufacturers: Sony and Pentax included). While both offer a lot of choices, one or the other is likely to better meet your needs. You will need to study each of them carefully to decide. Make a list of what you want and then look up what each manufacturer offers, costs, etc.

Since you're basically planning to start over from scratch, this may be an opportunity to switch from DSLRs to mirrorless, too. There are advantages and disadvantages to both types of camera systems: DSLR versus mirrorless. You should educate about them. Probably the biggest drawback to mirrorless right now is a shortage of native lenses and specific accessories. Both Canon and Nikon have introduced full frame mirrorless cameras within the last year... Canon EOS R has essentially the same 30MP image sensor as the Canon 5D Mark IV... Nikon Z7 uses a 45MP sensor similar to the D850's. (Both manufacturers also offer lower cost 24MP variants: Nikon Z6 and Canon EOS RP.) Both systems have relatively few lenses made specifically for their cameras, so far. But a lot of lenses are "in the works" and both manufacturers offer adapters that allow their extensive lines of DSLR lenses to be fully used on the cameras. (Canon offers choice of several different EF to RF adapters, including some with added features such as a programmable control ring or a filter drawer.)

Among mirrorless, Sony currently has the most fully developed full frame system.... But, again, you will need to carefully evaluate if what they offer actually meets your specific needs. For example, AFAIK, they have no f/1.2 portrait lenses or tilt-shift/perspective control architectural lenses.

Okay, this question has probably been asked a thousand times, so now 1001. And I’m sure I could find the answer on here with minimal searching but I’m asking again anyway. I’m not kind to trolls so please just skip to the next one if you aren’t friendly!
I’ve decided to move up in the world and say good-bye to my trusty Pentax, which has become an extension of my arm. I shoot portraits, real estate, classic cars, landscapes, etc and I make money doing it so I can justify spending or saving more and then spending. I’ve decided to go down the Canon or Nikon road; I’ve researched the D850 and it looks awesome. Is there an equally or slightly less awesome camera for a smaller budget? Please let me know if you have a D850 or different camera and what you think from your personal experience. Thanks for reading my long-ass question and for the (decent) responses I may get.
CCP

DO NOT RENT BEFORE YOU BUY.
Unless price is no object.
Renting a unit long enough to really see if it is a fit will significantly reduce your budget. Rental of a D850 with a 70-200 (portrait lens) and a 14-24 (real estate and landscape lens) will run you nearly $400 a week. Given the range of the OP's interests let's say 2 weeks for $800 or 1/3 of the cost of the D850.

Retailers like B&H and Adorama understand this. This is why they have very easy 30 day return policies with no restocking fee in most cases.

Pros rent for specific purposes for specific jobs. And pros can deduct the cost from their taxes. Renting makes sense for pros. For the rest of us, not so much.

DO NOT RENT BEFORE YOU BUY.
Unless price is no object.
Renting a unit long enough to really see if it is a fit will significantly reduce your budget. Rental of a D850 with a 70-200 (portrait lens) and a 14-24 (real estate and landscape lens) will run you nearly $400 a week. Given the range of the OP's interests let's say 2 weeks for $800 or 1/3 of the cost of the D850.

Retailers like B&H and Adorama understand this. This is why they have very easy 30 day return policies with no restocking fee in most cases.

Pros rent for specific purposes for specific jobs. And pros can deduct the cost from their taxes. Renting makes sense for pros. For the rest of us, not so much.

Okay, this question has probably been asked a thousand times, so now 1001. And I’m sure I could find the answer on here with minimal searching but I’m asking again anyway. I’m not kind to trolls so please just skip to the next one if you aren’t friendly!
I’ve decided to move up in the world and say good-bye to my trusty Pentax, which has become an extension of my arm. I shoot portraits, real estate, classic cars, landscapes, etc and I make money doing it so I can justify spending or saving more and then spending. I’ve decided to go down the Canon or Nikon road; I’ve researched the D850 and it looks awesome. Is there an equally or slightly less awesome camera for a smaller budget? Please let me know if you have a D850 or different camera and what you think from your personal experience. Thanks for reading my long-ass question and for the (decent) responses I may get.
CCP

Okay, this question has probably been asked a thousand times, so now 1001. And I’m sure I could find the answer on here with minimal searching but I’m asking again anyway. I’m not kind to trolls so please just skip to the next one if you aren’t friendly!
I’ve decided to move up in the world and say good-bye to my trusty Pentax, which has become an extension of my arm. I shoot portraits, real estate, classic cars, landscapes, etc and I make money doing it so I can justify spending or saving more and then spending. I’ve decided to go down the Canon or Nikon road; I’ve researched the D850 and it looks awesome. Is there an equally or slightly less awesome camera for a smaller budget? Please let me know if you have a D850 or different camera and what you think from your personal experience. Thanks for reading my long-ass question and for the (decent) responses I may get.
CCP

Okay, this question has probably been asked a thousand times, so now 1001. And I’m sure I could find the answer on here with minimal searching but I’m asking again anyway. I’m not kind to trolls so please just skip to the next one if you aren’t friendly!
I’ve decided to move up in the world and say good-bye to my trusty Pentax, which has become an extension of my arm. I shoot portraits, real estate, classic cars, landscapes, etc and I make money doing it so I can justify spending or saving more and then spending. I’ve decided to go down the Canon or Nikon road; I’ve researched the D850 and it looks awesome. Is there an equally or slightly less awesome camera for a smaller budget? Please let me know if you have a D850 or different camera and what you think from your personal experience. Thanks for reading my long-ass question and for the (decent) responses I may get.
CCP