Well see Apple doesn't sell 1440p external monitors, so the answer is you should stop using that trash and move to one that brute forces the sharpness problem and costs way too much because it has an Apple logo on it
Cute, but irrelevant. I just searched Amazon for "4k monitor 27 inch" and there are any number of highly rated screens from recognizable brands for around $200.
You don't have to splurge to get a 2160p monitor that a Mac will love.
Those monitors kind of suck. You can only get $200 if it's a crappy VA panel at 60hz. My 1440p monitor is a lot better, but apparently not supported by Mac for ideal rendering.
Because they want to sell you a Retina(tm) monitor. I wish I was making a joke.
Roll back to a version of OSX that predates Retina, and _all_ of your monitors get the expected Mac-like font rendering, Retina or not. Go to 10.7 or newer, and all monitors are ran using the Retina tuning for font rendering, which makes it very smeary and blurry on normal monitors, but looks great on anything that triggers Retina rendering.
So, what I've been advising to the fewer and fewer Mac owners I know that want multi-monitor: only buy 4k monitors, OSX thinks they're HiDPI and won't fuck over your font rendering. At least, they won't today.
Ever heard of subpixel rendering? You can have pretty sharp text at even 90 ppi if your OS supports it. MacOS doesn't, probably because they didn't want the complexity of supporting it throughout their compositing/graphics stack, and also likely because Apple doesn't sell any low ppi displays.