I think an iPad mini could be frustratingly cramped for what you're describing; smaller digital tablets feel limiting in a way that same-sized paper notebooks don't. Honestly I'd recommend an iPad Pro for this. It's expensive, and larger, but in my experience, I need that extra space so my mind can open up and I can just draw without constantly hitting the edges.
You might want to test-drive an iPad Pro + Apple Pencil first to get a sense for what the drawing experience is.