Is there a good 5k monitor at 27" that does not burn the wallet? It's worth mentioning that it should be also very reliable because these monitors seem to have issue after awhile, especially burn-in.
If you're not too worried about warranty etc, you could buy an old 5k iMac and convert that to work as an external display. I've converted a dozen now - highly recommend! If you aim for a 2017+ year model, you'll have a pretty reliable display. The 2015 models tend to have issues with red banding on the edges which gets super annoying when using a text editor with white UI.