> The same with opening theory: when I'm using my opening preparation, I'm playing at GM level as these are just the moves played by GMs in that position, and I did not need to find/calculate them, I just know them.
I wouldn't count that as playing the opening at GM level unless you understand why GMs play those moves.
Around 1990 there was a chess teacher and coach named Richard Shorman that would come to a public chess club that met weekly in Sunnyvale and give free advice to people who had hit plateaus and just couldn't seem to get better no matter how much they played and studied and analyzed. People would show him their games from recent tournaments and he'd analyze them and give advice to get unstuck. This was all out in the open so even those of us who had not brought games could watch and learn.
The people attending these sessions typically ranged from beginners who if they were rated were somewhere under 1000 USCF all the way to people in the 2000-2200 range who had been stuck in that range for years.
On of the big problems Shorman found with pretty much everyone there was that everyone wanted to play like a Karpov or a Kasparov. They studied the opening such players played, memorized all the variants of those openings from ECO, bought and read books on those openings, and studied the games with those openings from top tournaments.
So yeah...they might play 25 moves of a game just like Karpov or Kasparov would have because they are copying from a Karpov of Kasparov game that followed the same line. But what happens when their opponent plays a bad move? If it is so bad there is an immediate tactical refutation maybe they find that (especially if they are around 2100). But at the Karpov/Kasparov level there are a lot of bad moves where the move isn't bad for some short term tactical reason. It's bad because it gives some small weakness that a GM over the course of the next 20 or 30 moves can exploit to eventually allow some winning tactic.
And if your opponent does stay in your opening book to the end...then you just find yourself in a position that is supposed to be good for you, but without knowing why. A GM would know why they are better and how to use that.
It always does eventually come down to tactics, but the deeper you understand tactics the more you start to understand positional concepts and how they make it so certain tactics will or will not work. You can't really understand the positional stuff until you understand the tactical stuff. When you try to play like a GM too soon, you don't yet have the tactical skill to understand the positional stuff, and you get stuck.
One way Shorman put it was something like "Before you can play good chess you have to be good at playing bad chess".
For the lower rated players Shorman would tell them to play gambits. They might not be sound against high rated players but that's not who lower rated players are playing. They should be aiming for unbalanced positions and playing the most aggressive moves that they can't see a tactical refutation for.
As for books, what he'd tell the lower rated players to get was a collection of Morphy's games and skip to the end where it has the games where he gave odds or was playing simuls against amateurs--the games where Morphy needed to crush people.
That's what he meant by "bad chess"...the kind of chess people played in the 19th century.
For higher rated people, like a couple friends of mine who were stuck around 2000-2100, he'd still tell them to play unbalanced openings and play aggressively, but not in the balls out channeling Morphy way that worked for the lower rated people. Gambits were still recommended, but now ones that were not played at top level because the other side could equalize or get a slight advantage too easy rather than because they might actually be unsound.
That got both my friends off the long standing plateaus.
I was only around 1600, and wasn't playing tournament chess anymore (I was instead playing in Go events at the Palo Alto Go Club), so never got a chance to see if Shorman could get me unstuck, but I brought a list of my chess books to Shorman to see which if any I should actually read.
Younger chess players might not realize just how many chess books even casual chess players would accumulate back in the days before internet. Here was my list [1], and this was by no means a large collection for someone around 1600.
Shorman praised a few of them as good books that could teach a lot--and then told me to set them aside and read specific ones of them such as "My System" whenever I hit 2200, and in the meantime go get Morphy's games and skip to the odds games. (I never did get back into tournament chess, except for a couple of events).
I think Shorman's points and methods are still sound, but now with internet and online chess and computers that can automatically generate tactics training from real games we can probably go about applying them more efficiently.
We don't have to seek unbalanced positions and play aggressively in them in order to get tactical practice now--we've got tactics trainers. And now we can play more serious games against good opponents in a week then we might be able to get in a whole year of tournaments in 1990.
[1] https://pastebin.com/mw3q1784