All of that said, I've had the 10.5" Pro for 3 days, having made the decision to make an attempt at an iOS only professional workflow as well, buying it instead of upgrading my maxed 2015 rMB. The mini is keeping its position as my 'personal' device and since Wednesday last week, I haven't opened the lid of my rMB once...
But I've also only picked up my mini 2-3 times since then as well...didn't think I would like this 10.5" as much as I do. I find myself not wanting to put it down, like ever!
We seem to have a similar setup. iPad 10.5'' aside, I have an iPad Mini 4 and a 2016 MacBook. To be honest, I don't see anything wrong with this setup - I use the mini for content consumption at home and take it with me when traveling (usually along with my MacBook, unless I know for sure that iOS will suffice). Other than that, I don't usually take them with me, and just use the iPhone instead when out and about. These two devices are clearly differentiated. One has a small screen, the other has a decent production-oriented screen. One runs macOS, the other runs iOS. One does a great job when it comes to content consumption but a terrible job when it comes to production, and vice-versa.
Now, partly because of the novelty and features that it brings, partly because I wanted to experiment a bit with a new device/workflow, I decided to give the 10.5'' a shot, if anything, again, just for the fun of trying it out - still not sure whether I'll keep it or not. I absolutely love using it. The device is small/light enough for it to be reasonably portable while still making it more productive. In terms of video consumption, this iPad definitely beats the Mini. And, of course, I have the possibility of getting the Smart Keyboard, Apple Pencil (neither of which I'm sure I'll end up getting), it's got ProMotion, a better sound system, better specs, etc.
The problem however is that I can tell there are certain redundancies in this setup, one way or another. On the one hand, you could argue the larger iPad is not necessary if you have a MacBook and a Mini; the MacBook is almost as portable and much better for working, and the Mini is more portable, and in terms of content consumption it's usually reasonably close, sometimes better (e.g. reading) than the larger one. On the other hand, I could also get rid of the mini, sacrificing portability a bit, and gaining a lot of features and the possibility to do some work on the 10.5'' (especially after iOS11 is out). If you stretch it a bit, it might even be that the retina MacBook is redundant, and that I could simply stick to my work MBP (which currently never actually leaves the office) for serious, stationary work, and use the 10.5'' for everything else.
I guess I'll have to wait and see, but I'm really struggling with this one

. I'm almost in the middle of my 14-day return window, so I'll keep heavily using it in different scenarios.