It's interesting to read your comments, because there is some overlap with my situation. I bought a MBP 16 when it came out and I like to dabble in graphic design. I had an iPad Pro 9.7" with Pencil and love some of the apps, but I spend too much time on forums and not enough time actually creating things. Last year I had need for an on-board GPS (we had bought a used boat a couple of years ago and I wanted to use the Navionics app on the iPad and delay upgrading the boat electronics), so I bought the latest iPad mini. I figured that the iPad mini would actually be a great size for using Affinity Designer, as you can just pinch to zoom in for more detailed work, right? But I found myself having to pinch-to-zoom a lot more frequently than I wanted to (e.g., for editing nodes, and in particular to accurately select a particular node), and I also found that expanding the sidebar controls took up a lot more screen real estate.There were a few years in between that iPad and the first iPad Pro where I had given up on the device as something I could ever use as my main machine, so I just got an iPad mini and went back to my Macbook.
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That remained true until I started really jumping in to the Affinity apps, LumaFusion, ProCreate, etc. Those apps are all just way better and way more usable on a bigger screen. So this year I jumped up to the 12.9" model, and it's pretty liberating to have all that canvas.
In the mean time, my daughter was majoring in Studio Art, focusing on traditional media (and printmaking in particular) and had zero interest in graphic design, but I really wanted to nudge her over to building up a skillset in graphic design, and hoped that if she put some effort into it that she might actually love it. I let her borrow the iPad Pro 9.7" and she had been using it a bit, and raised some interest in the Cintiq, but I really felt like the iPad w/Pencil was the way of the future. She specifically pointed out that she wanted a very large canvas size. So, I told her that I might be willing to get her an iPad Pro 12.9". But I was still worried that the 12.9" would be too big and that she might use the iPad for more things if she went with the 11". But she has a 2018 (?) MBP 13" and seemed convinced that she would likely just use the iPad for drawing, and use her MBP for everything else. I'm not convinced that it will turn out that way, but I went ahead the other day and purchased a 2018 12.9" from Apple's refurb store. Her 12.9" arrived today. She's completed her semester and the school decided (last minute) to do some sort of virtual ceremony this Friday (they had previously pushed off the "real" ceremony until December), so I'm going to give her the 12.9" after that on Friday.
I'm still considering also purchasing an 11" so that she can compare the two first-hand, and also because I'm intrigued by these 2018+ iPad Pros myself, and may want to upgrade to one of them myself. Or I may stick to the mini for now, or move back to the older iPad Pro 9.7". My wife wants one of the iPads, though, and I'm not sure which she'll be happier with (she uses an MBA 13 as her main "device").
So in all of our cases, the iPad is a supplemental device to our MacBooks. In the case of my wife and my daughter, though, I suspect that they both could be pretty content with *just* an iPad along with the Magic Keyboard/case. I actually floated that idea past both of them and they rejected it, but I suspect that it's just because they don't realize how much you can do with the iPad Pros now, and how well the Magic Keyboard/case would turn the iPads into viable laptop-replacements. It won't surprise me if, in a few months from now, my daughter is using the iPad primarily and her MBP 13 is getting little use.