I'm trying to work out if you problem is flicker or not.
Are you sure it's not the glass screen that may be the problem with the mac?
Or it could be what you don't like is IPS glow. Or it's slightly higher response time. The response times for IPS panels are a lot better than they used to be, but some people do still notice it making it blurry when you move something on screen.
As for the top end EIZO and NEC they are high end for colour reproduction and accuracy, which doesn't appear to be the problem you're trying to solve.
So it might not be the resolution that is the problem, but the type of screen.
The IPS option I would go for has been mentioned the Dell P2415Q, this has an Anti-glare coating like most of Dells workstation monitors, the imac screens look nice, but I'm glad I don't have to work on one for long periods, (only when I cover for someone).
Or you could go down the high end TN panel route or VA panel. You won't get quite the same vibrancy of an IPS panel with a TN, but the top end TN panels are a lot better than they used to be. But what you do get is no flicker, and the fastest response time possible. Pretty much no smudging/blur or glow.
The VA panels are close to IPS in terms of colour reproduction, but have closer to TN panel response time. When they say 1ms response on a VA it's probably not as good as a 1ms TN, but very close. The way response times are measured can be a bit woolly.
Going down this route you would probably be limited to 2650 x 1440 res and 27", but you can get much higher refresh rates. 120-165Hz.
At the end of the day looking at them running is the best test.
My Dell S2716DG is very sharp, no glow, amazing refresh, colours are still good, just not IPS vibrant. It has G-sync which won't be any help for your mac, but you can still use a g-sync/free-sync monitor with having a compatible GPU (I have crossfire RX480 in my rig atm). And you don't get any blurring.