My goals with this secondary display is mostly websites / youtube tutorials on the side with my main coding/productivity happening still on the 14in Macbook Pro.
For that sort of usage, scaling issues are going to be totally irrelevant. The M1 Pro shouldn't break a sweat with scaling (unless you're already running heavy GPU loads) and the "artefacts" from non-integer scaling will be imperceptible from normal viewing distances on youtube videos and text. Not quite as pin-sharp as a 5k (there's no getting away from the 5k iMac having a
very nice display all-round) but a lot clearer than 1440p.
If you had a 27" iMac going spare, your idea would be a good one - but it's really not worth buying one for (esp. a 2020 model - still a perfectly good machine commanding good second-hand prices from people who aren't yet ready for Apple Silicon) when an external display would be more generally useful. Unless, of course, you've got other uses for keeping an Intel Mac around.
Universal Control does not make a computer (iMac in your case) an external monitor. It is purely for dragging and dropping files, etc. between the Macs/iPad.
...which is exactly what would be required in this case - to operate both machines side-by-side from one keyboard/mouse and use the iMac for browsing tutorials/videos. Before Universal Control, I've often used the third-party product
Synergy for this.
I came across a video a few weeks ago that talked about why a 4K monitor is actually NOT a good idea. The guy said to just go with a 1440p monitor, because of how the scaling works.
I'd beware of some of these YouTube "4k displays are useless" videos (especially the one referenced here with the silly red/green charts that suggest a 1440p 34" display is going to be "better" than 4k) - the points they make are valid but rather narrow-minded based on a few use cases that hit the limits of 4k displays. 4k
is a compromise compared to 220ppi displays and there are issues to be aware of. For
general purposes, though, 4k is a sensible compromise and will give you a clearer, sharper display than a 1440p at a fraction of the cost of a 220ppi display.
In this case, I think that guy's problems have little to do with scaling and more to do with the fact that his M1 Mini just isn't really up to running Blender at 4k or 5k
at all. Scaled mode should be a non-issue with Blender - which renders its own UI and where literally the first item on its "interface" preferences is the UI scale setting. Just put your 4k display in "looks like 1920x1080" mode, full-screen Blender and adjust it's own UI scale to ~0.75 to get an iMac-like UI scale. However, if you watch the video he says it was still stuttering in that mode (which means he'd likely have the same problems on a 5k display).
Given those circumstances - your main application struggles to drive 4k - then, yes it might not be worth paying for 4k.
The other problematic circumstances would be if you do a lot of image editing at "actual pixels" scale and lean in to see the individual pixels (personally, I'd just zoom in, but...). Which you can't really do in "looks like 1440p" mode - in which case just take a few seconds to switch to "looks like 1080p" mode, where you can, at the expense of a slightly over-large UI (or, if you've got nice youthful eyeballs, just use native 4k).
Again, i
f that's 90% of your workload, you might want to consider whether you'd be better saving a few bucks and getting a 1440p display - but while there are compromise workarounds for pixel-accurate editing on a 4k display there is
nothing that will make a 1440p display show 4k worth of detail.
Personally, I've got a nice dual 4k+ setup and £700 change c.f. the cost of a Studio Display.
