There is No General Strategy
Warhammer 40k is not a place where you can have a one-size-fits-all strategy.
First let me qualify that by saying there are general tactics. Knowing how many dice to throw at a psychic power to make sure it goes off or ringing your guys around an objective so your opponent can’t get within 3” are certainly general tactics any player can use. There is also good general advice like "ignore a gargantuan or superheavy if you can’t kill it". What I mean is that when you are constructing your list, having a set battle plan for psychic powers you roll on, where you deploy your guys, and what you need to kill isn’t just unrealistic, it’s shooting yourself in the foot.
The most obvious reason for this is that 40k is an inherently random game. Even a 2+ rerollable dice fails once in awhile, and you have no control over when that failure will happen. One of the biggest parts of being a good player is the ability to adapt to these rolls, whether they’re good rolls by your opponent or bad ones by you. If you have a set plan in your head for how a game will go before you start playing it, it is almost guaranteed that every single game something is going to go differently than it “statistically” should, and if you are stuck in your laid-out battle plan, you’re going to have a very hard time adapting.
The second, and more egregious reason that “general” strategies don’t work, is that they rarely account for your opponent’s actions. Assuming you know what your opponent will do is an insane assumption to make, because those guys can be crafty or stupid (it can be hard to tell the difference sometimes). I hate when people play out those theoretical games for me, where they tell me what I’ll try to do and why their list perfectly stops it. This is also why a lot of alpha strike and gimmick lists don’t hold up at the top level, smart opponents aren’t going to just deploy all of their important units on the line and let your Skyhammer drop in and kill them.
Lastly, the game is a very diverse place right now. As much as people complain that certain armies or play styles are weak, the competitive meta has grown wildly in the last few years, and you can see all kinds of crazy lists. Having a generic strategy for your army means you will inevitably meet an army that has a perfect answer to the way you play every game, and you’re going to find yourself beaten before the game even starts because you were unwilling to adapt.
If all of that is true, what is the point of planning at all? Why not just throw a bunch of random units together and hope the gods of random chance are on your side? Obviously there is a place for planning, but it’s in what your army does, not how to play it. Like I said in Play Your Game you can and should look at the missions you will be playing and think about how your list can win with those. Looking at the game in a vacuum is usually bad, but it's reasonable to look at a scatterbike list and say “I have a lot of very fast obsec units, scoring objectives should be pretty easy”. Also, while you can’t plan your opponent’s moves in advance, you can make sure you can deal with most kinds of threats. If you are missing a way to kill a gargantuan creature so you put a warp hunter or grav cents in a drop pod in your list, that’s good planning. If you assume your opponent will start their wraithknight where those units can kill it turn 1, or even that you will kill it turn 1 if you can hit it, that’s making assumptions you shouldn't make. Unit synergies are also something you can look at when planning your list. Some psykers to help buff your star or a guy who gives you reroll reserves when you have a flyer formation are good choices for making sure your stuff does what it needs to do. You just need to make sure you aren’t doing something like deciding which psychic table to roll on before you even know what you’re playing against or if you always want your flyers in on a certain turn.
I hope this all makes sense and if you have any questions feel free to ask. If you have a tournament list that includes Space Marines or Dark Angels, my list review submissions are still open as well. I should be back to posting these regularly on Wednesdays or Thursdays as of next week.