My opinion: iMac 2020 still has latest macOS and can bootcamp to be a great Windows computer too. If it's all working, stick with it until it can no longer keep up (which will be when Apple decides to stop supporting Intel Macs)... possibly as soon as this Fall or maybe next Fall... giving you at least about 7 to 18+ months before you really need to do anything.
When that day comes, since you need REAL Windows for that laptop, don't even consider ASD as it is finicky about what you connect to it. Instead, go with
any of the other options. I suggest specifically seeking out a 4K+ monitor with at least
TWO video inputs, so you can allocate one to your next Mac and one to your Dell.
If you buy the "5K or bust" hype "we" sling around here as if 4K looks atrocious (and it doesn't per my own 20:20 eyes), consider
Samsung Viewfinity S9 for this, sometimes on sale for under $1K and it comes with all of the key stand options that cost about $200 extra on ASD.
I faced the very same dilemma in going desktop Silicon but needing real Windows (not ARM Windows) too, so I chose the
5K2K Dell 40" Ultrawide (an
updated version is due to be released soon with some good improvements). It features
FOUR video inputs and has a pretty loaded hub built in too, making it easy to share most fundamental accessories (like speakers, keyboard, mouse, other hubs, webcam, printer, etc.) between Mac and PC depending on which one is in control at the time. A bonus is that since it is so wide, it can easily split the screen so that I can have Mac on one half and PC on the other when it is useful to have- and see/use- BOTH at the
same time. This Dell screen itself looks just like my old 27" iMac but with the screen stretched much wider and the chin finally jettisoned.
There's also the
LG 5K monitors, which were the ones Apple endorsed by stocking them in their own stores until they rolled out ASD. So they were fine
BEFORE ASD but then "plasticy" and "wobbly"
AFTER Apple rolled out a competitor. But it too is a
great monitor for substantially less than ASD in general. However, I think it is also a single input monitor, so you would need to swap cables regularly to use it both ways.
I chose Mac Studio instead of Mac Mini but that's because there was no PRO M-series Mac Mini at the time. If I was back to making the decision vs. the PRO Mini option, it would be a much more carefully considered one. If the 2020 iMac feels fast enough for your uses, EITHER should be perfectly good as a new Mac for you.
So nutshell: ride the iMac 2020 until the wheels start falling off, then consider a monitor that fully serves
both worlds, which is not really ASD.
I hope this is helpful.
One more idea: consider moving the desk/workspace to somewhere else that WOULD accommodate a good-sized monitor for the Dell to put beside or above/below the iMac. There's very likely another spot somewhere around your place where a dual monitor setup would work fine. And Windows works well with pretty much ANY monitor, so you don't have to look at Apple-like budgets to get a great monitor for that Dell. Thanks to robust competition, there are
very high-rated 4K monitors for only a few hundred dollars.
And if you want to go "all the way" now, buy yourself a NOT-ASD monitor with at least 2 inputs, and a new Mac Mini or Mac Studio will be a big leap vs. the 2020. Give iMac to your wife and enjoy your new setup: both latest Silicon Mac and that Dell PC as "old fashioned bootcamp" sharing a
single monitor and key accessories connected to the hub on that monitor so that BOTH platforms can utilize them.
