Pook: buy the 21" OLED 4k and custom electronics to drive it
Live happily ever after. You don't need OD or anything for that. There were those 4k120 / 1080p480 electronics. But it's a pure DIY and not cheap at all. Yeah it's sad that we are at a point where with enough money you can sometimes buy better parts separately to build a monitor than a finished product from companies.
Depends, if the panel in 31.5" 1440p has same connection and communication as 27" 1440p then you could use the faster electronics from 27" models on a 31.5" panel. But unless you own and open both or find details for both, you will never know.