I am considering whether to get 2x30-ish 4K/5K or one big 43".
...in the absence of any replies from people who have made that particular switch...
As a fan of dual screen set-ups, it can change the way in which you use the space. I find "window management" a bit easier with two displays - e.g. its really easy to full-screen one window on each display, or sweep occasionally used windows onto the other display. So if, e.g. you're coding, you can have your IDE full-screened on one display, and your website/app/whatever, documentation etc. on the other. Or, for video work, your timeline on one screen, the full-screen video preview on the other. Or MacOS on one screen, Windows in a VM on the other. Or (if you're that way inclined) work on one screen, TV playing on the other. You can set up MacOS to have a different set of virtual desktops for each screen.
With a huge, single display, you may find yourself trying to tile windows to keep everything you need visible (there are tools to help, such as BetterSnapTool, or MacOS's built in split screen mode).
OTOH, consider that you probably won't use dual screens with your nose pointing at the join: more likely, one will be your primary screen, directly in front of you, the other will be secondary, to one side of it. If you get two 43" screens, you might need binoculars to read text on the far side of the secondary screen.
Note that MacOS is quite happy with different sized/resolution screens - a vast mismatch makes it slightly odd when dragging windows from one screen to the other, and (in multiple-spaces mode) you can't leave windows straddling the screens, but that's about it. I'm mainly using a (lovely) 5k iMac for the main screen and a cheap-ish Dell 4k 28" for the secondary.
It depends on your workflow and why you want such a huge screen area: if you want to edit huge swathes of text or massive spreadsheets - or maybe you do everything
in a single application with its own screen tiling - then a single, large display might be better. If you're interworking with several applications and lots of files, dual-screens might be better.
Question is, what are you using at the moment?