Speaking as someone who's in the market for a new display, I haven't been able to find a quality 27" 5K screen. LG has one, but it gets poor reviews. Everybody else is selling 4Ks, which is lower effective resolution than what I'm using now (a 23" 1920x1200 Apple Cinema display). So this is definitely appealing to me.
The effective resolution of 4K is 2x in both directions because to match the size you just set the 27" screen a bit farther away so it fills the same field of view. The jump in resolution from going 4K will be very noticeable and a 27" 4K IPS screen is less than 300€. Well worth the upgrade.
I am in the same bucket. LG Ultrafine is the only option which seems available. But it's an old model and I am surprised the lack of availability from the other manufacturers.
I have the LG ultra fine 5k and it's a very good monitor. It has a small defect: occasionally there is a small flickering on the central vertical line. It goes away if you put the screen to sleep and wake it again (it takes a few seconds, and I had to do it a handful of times in a year)
