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I guess you should prepare openings at least as white, you should know the idea of opening you play and what your opponents play, like in sicilian black aims at getting d5 push. In london if black gets e5, white has not played london at its best. Puzzles also helps you in tactics seeing faster and grabbing pawns, only if you do lot of them it gets into your mind. Endames can be learned from yt once you encounter them in your games.

You can check out custom puzzles to go one by one and get perfect in each section of tactics.

If you are rated under 1000, learning openings is probably the least useful thing you can do.
The most useful is solving puzzles. (I mean actually solving them, not just guess a random move and go on). The second most useful is watching chess videos.
If you want to buy a chess book, don't buy a book about openings, it is going to be boring and also most likely seriously out-of date. Buy one about middle games or tactics.

Theoretically, the more you calculate the better. This is the best (and perhaps only) way to improve your calculation skills. In practice, staring at it for a long time is not productive, so after 10 minutes I usually make an educated guess. (I guess it is very different for the vast majority of players, who make an educated guess after 2 seconds).
#1
Always check your intended move is no blunder before you play it.
That alone is enough to get to 1500.
Most important is to play and to analyse your lost games so as to learn from your mistakes.
Solving tactics puzzles is good, but it is no substitute for real play. In a real game nobody tells you that there is a tactic, or for which side.
Openings are not useful. Just play on principles: develop your pieces into play and control the center.
Endgames are important as fundaments. First study 3 men, then 4 men, then 5 men.

Before you play any games for the day I'd suggest using your diamond membership to go to the lessons and find the lessons that say "play like (GM name)" pick whoever you want and play through one or two of their games. When you play through the games use the hints if you need to, tell yourself why you think they made that move, what's under attack, why's he attacking/focusing on this pawn? Try to get a general understanding of basically how they develop their pieces and make plans.

Improving Your Chess - Resources for Beginners and Beyond....
https://www.chess.com/blog/RussBell/improving-your-chess-resources-for-beginners-and-beyond