One thing in your post really stood out to me:
What other way is there to live than to treat others neither as masters nor slaves, but as trading partners (in love, friendship, mentorship, business)?
You're advocating the opposite of that. As a landlord -- assuming a world with absolute property rights -- you have a significant power advantage over your tenant. A 3x-4x rent increase is not treating your tenant as a "trading partner", it's using your status as landowner to your enrichment and their detriment.
Further up you suggest:
A group that votes to dictate what I can do with my property might be doing it by the letter of the law, but that doesn't make it morally right
Not inherently, no. That's only the case if you believe that the voting-majority group is immoral. That certainly can be the case, but I don't believe it's the case often enough to abandon the idea of democracy.
Personally, I'm definitely in favor of limiting property rights. Property is in general not a fungible "product", and its supply is limited. I consider land to be a public good. The state grants you some property rights, but reserves to place some restrictions on what you can do with the property in the name of the public good. I don't think we have the balance down perfectly yet, but I would absolutely not advocate giving property owners carte blanche to do whatever they want with that property.
It's a matter of drawing the line somewhere: I think you might agree that some restrictions are good. Disallowing renting decisions based on discrimination of protected classes is something we might agree is a good thing (sorry, I don't mean to state your position for you, but I don't know you, so I'm just stating a hope, a guess, as to somewhere we might find common ground). If we can take that as a given, then we can agree that the state restricting property owners to some extent does indeed make sense. And from there it's finding a list of things that it's ok to restrict, and it's not ok to restrict. From this discussion I'd guess that my list of things that it's ok to restrict is a bit longer than yours. And that's ok, but at least it's a starting point from which to find a compromise. The final list has more on it than you want, and less on it than I want.
Business deals and compromise are rarely about making one or the other party (or both) 100% happy. I believe there's "common wisdom" floating around that the best deal is one that both sides are slightly uncomfortable with, and I believe that to be pretty true.
If, on the other hand, you do truly believe in giving property owners absolute power and free reign to do whatever they want with it, then I don't know that we can actually have a productive discussion, because our viewpoints may just be too different. That's unfortunate, but that happens sometimes.