Now a days I think the tactics streak offered on Lichess (and probably on chess.com as well) is a really great tactical resource. It's the untimed option, rather than the much more popular tactics rush where you have e.g. 5 minutes to do as many as you can. I think the 'woodpecker' method of tactics is a great idea. Basically you build up on a repeated series of tactical problems, until you're able to complete them all perfectly accurately at an extremely high rate of speed. This seems contradictory, because the sites have massive tactics database, but you will regularly see the same problems due to the birthday paradox. If you're looking for some achievable ballparks, on Lichess I tend to be able to hit around 50 somewhat regularly, with a high of about 90.
I think the fundamental thing with openings is that so long as you make logical moves, even if you make an objectively weak move - you will very rarely reach a losing position because of it. And from that point on both your opponent and you are both out of book, so whoever understands the position better, and plays better, will win. Many people, especially adults, get caught up in obsessive opening study because it's the one form of chess study where results can be immediately felt.
The one very good thing about opening study is seeing ideas and concepts that you may not otherwise be able to come up with on your own. Like in the Najdorf, the pawn structure with d6+e5 and d6 generally firmly blocked and on a semi-open file leaves d6 feeling like a major weakness at first. The fact that it's generally rock solid was a serious eye-opener for me! And that translates strongly to many other positions - backwards pawns are not necessarily dooming one to passive defense, and can even be a great dynamic weapon!