The price of 4k discs will mean very little compared to the price of a quality TV set up to display the potential of the discs properly. At the very least you'll want an OLED panel tv, a budget model would be something from LG like the C2 or older C1. The Sony A95K and Samsung S95B are the big dog tv's with the newer tech QD-OLED panels. You're going down a long road of research on how much to spend on a setup, so back to my main point spending $10 or $20 here or there on a disc is really a non issue compared to the decision of spending $1k-$3k on a tv. In addition to that you'll need a disc player so Xbox Series X, PS5, or standalone player for non-gamers is another $500. Discs are always at their cheapest around black friday which just ended. Brand new titles were selling for $10 or less. It's the best time of the year to start a collection.

It was clear to me in your first sentence you don't own a 4K HDR tv. As a photographer myself I'm very aware of the trick of using different exposures to get different brightness levels in the shot but let me tell you HDR on an OLED tv really has nothing to do with photography in the way that you are comparing it. Changing exposure levels in different areas of the image is not what makes 4K HDR great. It's not just exposure level it is color depth and color contrast enhancements that make the huge difference.

You're comparing apples to oranges by saying your projector is more than enough for your needs. That's a choice for you, but that doesn't make it correct advice to give people. Walk into an actual store with a 4K HDR demo disc on a QD-OLED TV like the Sony A95K or Samsung S95B and tell me you can do that on a projection screen with a standard bluray and no HDR. It's not possible. Is it "Good enough" not to have those things? Maybe. Some people don't care. Some people still think DVD looks great too.