Yeh I think the answer here is that its gonna be different for every person. On my server I cant even get into a Naxx pug with a single character because I dont have the achievement to link them already -.- I am fresh at 80 though so give me a few weeks of gearing and im sure Ill be making the same decision about wether I want to spend my time in a PUG or do it the hard way and go solo all the way. Personally Im thinking Ill just do a bit of both depending how tolerant im feeling on a given day

Edit: One thing I definately want to do when Ive got some gear though is go back through as many wow classic dungeons and raids again for nostalgia purposes, then possibly move onto tbc stuff, tho I think ive seen most of that enough to last a life time!