Dual monitors work really well in situations where you read from one program and work in another. Some photo editing can be done with a viewing screen and a controls screen. If your editing programs don't do that then you might want a larger monitor.

For editing portrait images, vertical screen height is a big plus. I find a big difference between 1080 and 1200 vertical pixels with both being on 24" monitors.

If you are generally working in one program, then a single large screen likely makes the most sense.

I haven't used or viewed 4k, but I'd think screen size in inches is still more important up to 30 inches or so. As you get larger, then extra resolution becomes more apparent. For that budget, I'd look at screen size and quality first before 4k, but each of us has different needs.

One other detail. Since I wear progressive bifocals, I find screen height on the desk a factor. It's tougher on my neck to read near the top of a tall screen these days.

Hopefully, I've given you a few questions to think about, even if I don't have any answers for you.