Great tip about the sandpaper! and yeah automotive finishes are really nice, you can even go full blown and use them wet to reduce dust and to give you a much cleaner sand, you can sand and polish topcoats IF YOUR VERY CAREFUL NOT TO GO THROUGH THEM in this way! Another way you can make them flat is to tape them to those little dividers you get in plastic compartment boxes, it makes a nice large surface!
but about the pen vs wash, its just that a wash is so quick and simple that i think it is still beginner friendly (MORE beginner friendly then correctly using a pen i would argue) and I would rather they get into the habit of washing and gaining experience than starting off using select specially tools that probably do have their place but I think are counter intuitive to the experience, (we didn't pay money just to half ass our own gundams did we?) That being said, the hobby is all a journey unique to yourself, have fun doing what you love and like! I would just be amiss in recommending it lol