My story matches yours almost exactly. Didn't take a class, did a bunch of practice tests, got an almost perfect score.

I agree that confidence is very important, and aptitude matters too (otherwise everyone who took a class would get a 2400).

Commonly-given wrong advice is about guessing. You should never leave a question blank. Prior to 2016, guessing had an EV of 0, so if you had any hint that any of the answers are better or worse than chance, you should guess. I heard advice like "only guess if you can eliminate two answers", which is clearly wrong.

Now they got rid of the penalty for wrong answers, so guessing has positive EV even if you have no information at all.