I use the 12.9-inch, 128GB, wifi model of the 1st generation iPad Pro. I’ve always preferred the A4 paper size for my note-taking. It only made sense to get the 12.9-inch version because it is the exact size of an A4 paper. It is portable (some will disagree) yet big enough for mind mapping. I can cram a lot of information onto one page. The larger the screen, the better for mind maps.
Split view decreases the screen real estate available for each app to use. That is why I didn’t go for the 9.7-inch iPad Pro. I use my iPad as both my textbook and notebook simultaneously, especially when I am not at home. Studying on the go is much easier on the bigger screen.
I chose the 128GB model purely out of preference for it over the 32GB. However, I would have chosen 64GB. I’m hoping the increased storage will allow me to keep using my iPad Pro for longer. 32GB is all I have used so far for notes and textbooks.
To minimise distractions, I bought the wifi version. My iPad Pro is primarily a notebook. Paying an extra $300 for LTE is not worth it. It might even make me less productive. Most places I study from (my room, the library) have wifi, so an internet connection is readily available (most of the time). Find out which iPad you should get for your workflow.
All the reasons you mention for going paperless can be achieved on MS Surface Pro 6 too. I find MS Surface Pro product more conducive to student use because of the kick-stand, the keyboard and now with Surface Pro X, you can store and charge the slim pen in the keyboard:https://www.youtube.com/watch?v=v5SFBpMiaiQ&t=22s
There are many products on the market to go paperless now. What matters if you find one that works for you.