As someone who has returned to the hobby after "getting over adulthood" I would say that the tools/items that make the difference between childhood botch jobs and patient (ha!!) grown up finished products are: liquid poly cement (FWIW I've got Humbrol and use a v.small brush to apply), Squadron sanding sticks (how did I ever manage without??), Expo sprue cutters followed by a good scalpel (Swann & Morton for me), cutting mat and...yes I'm afraid...an
airbrush (thought I'd slip that in there, I got a £80ish starter kit off eBay, seems to work well for now)...
There's lots of other things of course (fillers, weathering, varnish etc etc) but this would be my list of things that have made what used to be a chuffing frustrating hobby into something I can honestly say immerses me for hours at a time. It's still frustrating at times of course, but having at least a few of the "right tools" helps massively!