This is the baby I’d go for… https://www.acer.com/ac/en/US/content/predator-series/predatorx35
When one of my dual 27" (2560x1440) monitors died, instead of getting another cheap one for $200 I spent $230 and got a cheap Vizio 40" 4k TV so have more pixels than my dual 27", but in a different shape (3840x2160). Then use Better Touch Tool shortcuts to instantly switch windows to various sizes/locations of the screen depending on how I want to focus my attention. I don’t get the picture by picture etc of @Norman Palardy’s setup, but do use screen share to simulate it. Either picture by picture, or in a separate mission control desktop I swipe to on my trackpad, with that desktop running the screen share in full screen mode so it looks and mostly acts like I am on that machine.
For the price is hard to beat.
Anyone considering getting a 5K or those wide curved displays: Beware the pixel density. They’re usually leading to the effect that everything appears smaller, in order to get more pixels on. So, if you want to work at exactly 1x or 2x (for the 5K displays), everything is smaller than on a “regular” display. But if you’re already using a modern MacBook, you’re probably working on such a high density monitor anyway.
My eyesight isn’t too good any more, and my ASUS 1920x1200 monitor still shows text at 72 dpi (1x). The 5K (an Iiyama) comes closer to a 96 dpi. Sharper, but also making everything smaller and thus harder to read for me.
Both displays have about the same size (nearly same height, but the 5K is a bit wider), but the Iiyama crams more pixels into it (5120 x 2880, or 2560 x 1440 Retina).
But both monitors next to each other, I can move stuff to the left for 1x and bigger overall appearance, or to the right on the 5K monitor for seeing more. Having both a 1x and a 2x monitor present also helps noticing any errors in Retina handling 