I don't need to "learn" about anything here—I'm quite educated about the topic. You're just putting an incredible amount of (and wasted) effort into what I can only assume is avoiding acknowledging that you know exactly what I'm talking about and it's absolutely true, 100% of the time.
We can puke up all the ppi, density, scaling, bit depth, contrast ratio, density, anti-aliasing jargon and stats all we want, but it doesn't change the fact that when you go from a resolution of 2560 pixels wide to 5120 pixels wide, everything on your screen appears at half the size.
I think you would be crazy to buy either of the XDR displays. The regular Studio Display is a much better value for you, even if you can afford either of the XDRs. As a photographer, you're getting absolutely nothing for double (or triple) the price. The only legit benefit of the older XDR is the size, but I can assure you that even thought it's a 6K screen, you will not be running it at 6K resolution - the entire interface of the OS, Photoshop, Lightroom (and every other app) would simply be too tiny to see. Even the 5K Studio Display is rarely used by anyone at the full 5K resolution for the same reason. The other consideration is if you want/need a webcam... the older XDR doesn't have one built-in.
My 2 cents: You're going to love using ANY combination of the 4 Apple displays... so it really just comes down to money and desk space. VESA Mount versions of any of them would be my recommendation.
Sorry but I think you really did not get the concept of resolution scaling for HiDPI Monitors.
You are stating that there is no benefit in using either a 5k or 6k display as running it at native resolution will make the UI elements to small. Running at native is truly not the way to go, however with enabled scaling you don't loose any resolution:
You don't have to run a 5k, 6k or even any HiDPI at native resolution to actually benefit from it.
UI scaling work by increasing the rendered size of UI elements, not reducing actual resolution.
So a typical 5k display has 5120 × 2880 pixels, 200% scaling appears as 2560 × 1440 pixels. This does not mean the monitor runs at 2560 × 1440 pixels, it's just a reference to pre HIDPI real estate jargon. It still runs at 5120 × 2880 pixels, one pixel just becomes 4 pixels, therefore UI elements get sharper with scaling, not just bigger.
So on a 5k display with 200% scaling, every pixel is used. UI Elements get more pixels AND Images, videos etc. are utilising every pixel independent of scaling.
To the Thread starter:
Honestly I'd go for the 32" 6k for photography if you work with high-resolution images and wan't to see as much of it.
For reference: The current 5k displays show you around 15 MP, the 6k around 20MP.
I've switched from 5k (LG, Apple) to 6k Monitors (XDR, Asus) and the added real estate is much more worth it than i.e. 120hz and miniled.
Go with the Asus 6k or similar (Kuycon..) if you wan't to stay on a budget. Avoid the LG 6k, it has a comparably bad matte coating. If it must be Apple for you, go for the XDR 6k. Maybe Apple will still release an updated 32", I'm so hoping for this, can't believe they killed the 6k at this point.