Purchasing and studying quality opening repertoires is useful for OTB tournament players who have ambition... also, they're almost completely useless for online blitz.
Of course openings are useful in speed games, but a few common lines with a few common ideas is more than enough. Many amateurs as well as top blitz players (like Hikaru) have their own blitz-specific openings (usually consisting of practical sidelines).
Yeah, these courses are much bigger than what an amateur needs to know. Start with the Short & Sweet and the Quickstarter to get a good overview and then you can dive deeper in the variations that are the most common in your games or that cause you trouble. You don't need to study every side line up to move 15 or 20. Many of them will probably never occur in your games
just play a lot, and do some chesstempo problems. (bonus if you analyze all your games).
How much openings to study is based partially on which ones you pick and your playing style. They are some openings that simply require hitting the books to not embarrass yourselves, others are so simple, that you can get away with natural moves. Your goal as an improving player is to get playable middlegame positions out of openings and to be exposed to a variety of positions.
Knowing those endgames is very useful.
I doubt though that studying all the sidelines of an opening will translate into rating points at the amateur level... I would focus on the 20% of lines you will really get on the board...
Opening study is a waste of time.
That what you study does not happen and when it does, you will have forgotten.
Games are decided in middlegame tactics, not in the opening.
A blitz player does not need much of opening theory, plus that the "spaced repetition" learning method of chessable is completely dumb and counterproductive.
I don't like chessable for exactly the reason you describe.
I do believe it is important to know good moves, common traps and the meta-game ideas behind an opening. And sadly, knowing those things just requires a lot of pure rote memorization. For example, you're not going to play a Maroczy Bind accidentally. You're going to have to take the time to study what that actually is.
Even with rote memorization, I dislike chessable because it shows you move 4 and you just have to come up with move 5. Chess masters tend to chunk moves together. They don't see
1. e4 c5 2. Nf3 d6 3. d4 cxd4 4. Nxd4 Nf6 5. Nc3 g6
They see Sicilian dragon.
So trying to learn the Sicilian dragon by showing someone 3. d4 and having them come up with cxd4 is just counter intuitive. And all the different lines they try to shove down your throat like this, Idk.
Some people seem to really like that site, though. I just don't get it.
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I bought two opening courses and the course for '100 Endgames You https://omegle.onl/ Must Know' by De La Villa (all on chessable). Aside from that, I spend quite some time on drilling tactics.
So far I enjoy chessable as a whole and the value provided by studying De La Villa's book seems evident. However, I am getting a little concerned if the time necessary to learn and retain(!) some 400+ lines per opening is a smart allocation of the time I spend on chess. Don't get me wrong, it's not that I am unwilling to put in the effort, it's more that I don't know if it will be of much use or if it's more so that the vast majority of lines will not even manifest on the board up to the really high rating realm.
My blitz ratings are 2000 on Lichess and ~1500 on chess.com