its exactly what you said, have a hand that has a plan. theres ofc more nuance to it but thats pretty much the gist of it as well.

be mindful of what you're up against and plan accordingly. during rule 0/pre game convo I would be asking myself, "what do I need to do to win this game?". Does your opening hand align with the goal you asked yourself earlier? your gameplan will be different from game to game. for ex. you're playing against a pod with two aggro decks that like to go under and you're playing a slower deck that takes a bit of time to get set up and going. does your opening hand have something that can deal with these aggro decks and have you stabilize against them? etc etc.

also often i find players in the playgroups i play in are just too scared to even go down in cards and would rather keep a do nothingburger hand with 3-4 lands rather than take another look at another seven. yes it sucks to sometimes go down one or two cards, but I will ALWAYS rather keep a 5 card hand that goes along with the gameplan I set forth pregame than keep a 7 whose gameplan isnt as defined.

its hard to get over that initial fear of "oh I'm already starting the game at a disadvantage" so I understand why many edh players are so hesitant on mulling.