Mac Mini can use keyboard and mouse for one thing, iPad can't do that as well - especially mouse. And iPad keyboards are somehwat meh. And that then leads on to all sorts of other awkwardness with using software.
However I see I didn't read your post properly - interested in how that setup works, but even if it works well it still wouldn't suit me because important-to-me programs aren't available on iPad, because iOS is not Mac OS etc.
Plus I'd have to keep carrying my iPad around and that doesn't suit me either... and my desk for my PCs is already cluttered (where the keyboards and mice are, and monitor). So, not really something I'd want to do. And sitting on the sofa with leads plugged into my iPad I find clumsy and annoying.
That said, I mostly use my iPad, because it does what I need most of the time. On the sofa. Otherwise, I fire up a PC.
I am however pondering leaves Windows behind so if an iPad could run Mac software and use a keyboard/mouse/monitor well - I'd be absolutely interested!