Personally, I don't think those basic 5e rules are very complicated. The actual "rules" section of the basic rules are about 25 pages, and even skimming there are a lot of things that won't really come up in play. About 60 pages are given over to spell descriptions and monsters. The game is complex because of its exception-based design. Things work one way unless you have a special ability or a spell, that then interacts with this other ability to produce this unexpected effect, etc.
But it's interesting because the "dm tools" consist in monster statblocks, a short selection of misc random items, and rules for building "balanced" encounters. For how sparse the rules are, and for how much they lean on DM adjudication, you'd expect maybe 10-20 or so pages of general dm advice plus scenario-building procedures. It's also absurd that the basic rules purport to cover levels 1-20. Is anyone playing 20th level 5e with just the basic rules?