I too have been looking into what can replace the 5K 27" iMac display, because my 2017 5K iMac display has developed a column of bad pixels about 30% in from the right side.
It's been that way for about a year and a half now and has not gotten any worse so I don't think I'm in any danger of suddenly needing a new Mac. The way my iMac happens to be on my desk I'm directly in front of a spot about 30% in from the left and most of my main focus is on the left side, so the bad pixels aren't too annoying.
The 27" 5120 x 2880 display has definitely spoiled me. I want something with a similar density and not much bigger or smaller than 27".
These seem to be the choices currently:
1. Apple Studio Display. As you've noted it is expensive. It also comes with a stand that does not have a height adjustment. Add $400 to get it with Apple's height adjustable stand. (Or for $0 get it with Apple's VESA mount adaptor instead of the standard stand, and buy your own stand. If you can find a VESA compatible height adjustable stand for less than $400 this is cheaper than buying Apple's stand).
One thing to not if like me you would really like a monitor that you can keep using for a very long time is that in 2021 Apple made a change to AppleCare. It used to be that you could buy at most a small fixed number of years of AppleCare. Now you can keep renewing AppleCare indefinitely.
For the Studio Display it is $49.99 per year. If the Studio Display is fine for someone except for the price, it might be worth considering if that plus $49.99 per year, which should let you keep it working for a long time, would make it worth it.
2. LG UltraFine 27MD5KL [1]. Supposedly this is the same LG panel that Apple used in the 5K 27" iMacs. Just under $1300. Seems to get good reviews.
3. Samsung ViewFinity S9 [2]. This is new. $1600 so same as Apple's price for the Studio Display, but the S9 is height adjustable and VESA compatible so no paying extra if you want height adjustability.
[1] https://www.bhphotovideo.com/c/product/1500040-REG/lg_27md5k...
[2] https://www.bhphotovideo.com/c/product/1760795-REG/samsung_l...