I bought a very slightly used Iiyama 5K monitor (XB2779QQS-S1 with Displayport 1.4) for 450€. While it does have some stuck pixels, they are so tiny that you only notice them when you are looking for them.
1800€ is insane.
I also use a 27" 4K monitor. The difference between the two isn't that great. 5K is mostly useful for 4K video editing. PS: It sounds as if the author would have had a great experience with a 350€ 4K monitor, too.