Subpixel rendering has nothing to do with any of this: it was messing up on non-RGB pixel layout panels like VA and OLED, and it used to be a simple setting in GNOME (hidden these days unfortunately).

Still, even 5K at 27" is not without noticeable jagged edges in diagonal lines and textual characters (though I've only tried 4K at 24", but that's a similar DPI and angular resolution if at the same distance) if your visual acuity (with or without correction) is around 20/20 or better (mine is better with glasses/contacts).

I hate how the text looks with a Mac on a 4K 32" screen, let alone 4K 42" screen.