I’m very interested to see what Catalina and iOS 13 brings in this space, I think just having it available to more people will see people find a use for it since plenty of people have both iPad + Mac. The touch tablet options using a pencil also have a lot of potential. Of course I also expect plenty of “because we can” sort of apps/uses to come out.
In my Windows life I have long been a dual screen fan for all of the reasons having a second display makes working more efficient. Even at home I have dual screens, although it’s fair to say they don’t get the same work out they do at work. The funny thing is that when I was using a Mac I never did dual screens, only a single 27" and never missed the duals.
I do wonder if that’s reflective of Microsoft’s “full screen” button and it’s ability to snap to half a screen, things I use all the time. Yet OSX has always been about floating windows you can resize. Again strange that on the Windows side I never really think about floating my windows around and having them overlapping.
But on your needs, there is plenty of pretty cheap 1080p monitors available at a range of sizes/budgets. Pretty much everything will work with OXS, but I’d make sure whatever you choose works with your iOS device. The real question will be if you want to fork out for a 4K model or not or if a FHD monitor will be enough in your otherwise retina world.