Well, if you're good at tactics, endgames, and you know 15 moves of theory, then...
... then you wouldn't be rated under 1000...
True, but under-rated is much more likely. We played 5 unrated rapid games together and I was lucky enough to win 2 of the 5! 
Well it's a bit misleading when the OP posts a game where they beat a 800 and asks about simple opening stuff 
But ok, good to know.
I was in an OTB chess club where the weakest player was rated 1800 OTB and the strongest one was around 2350. I was around 1000 OTB strength at the time and it was pretty hard on the ego to play against them because the outcome always felt obvious before even starting a game. I was outmatched in every aspect of the game and they were more competitive than the type to coach and explain how to play. I did get help to fix the more blatant weaknesses that really needed attention such as not accepting every trade and trying to get something beneficial out of it. I don't really blame them for spending most of their time improving their own games, it's quite normal. I spent an entire summer vacation trying to figure out how to close the gap in skill between us, but I just kept finding more things to improve on my side just to have a chance to play a few more moves before falling apart. I never managed to win against them so, I eventually stopped chess because I couldn't get in the right mindset anymore. What's funny is that I probably went from 1000 strength to around 1400 strength OTB in this period, but since I was constantly losing against them it felt like I never got better.
Fast forward in time, I spent a ton of time doing puzzles, but still neglected strategy and openings. I kept doing whatever I wanted in openings and relied heavily on tactics. I could lose a game in 11 move with a fried liver attack and most classical opening traps that aren't a mate in 4. So, I was still a lot behind positionally. At some point in time, chess got popular again on twitch and I started watching youtube videos as well of whatever was trending and I started looking up on the side openings because I felt like my positional play with certain openings was too unstable. You don't want to lose a game and ruin your chances in a game on the first 10 moves. I didn't really care about winning/losing anymore. I just wanted to figure out why every time I play a move, it feels like it's not good enough. I'm pretty sure I'm underrated, but I was probably also underestimated. I don't think I can maintain a 50%+ win rate against someone rated 1500+ if they play enough games against me.
When I was ~1500 I went to a club where the weakest person was 1800. Every week that I went, I had to try extremely hard if I wanted to avoid losing every game. That helped me improve a lot.
Would be nice if I could find a club full of 2200 OTB players and above to beat me up... too bad.
I can say you picked a very difficult opening strategically with e6 and c5 inviting a benoni and also a french or sicilian. You can think about certain pawn structures to reach and then choose the openings that reach them frequently.
The carlsbad for one, is very stable and can be reached from a number of main line openings, both stemming from 1 e4 and 1 d4.
The isolani is not as stable but still very frequent and relevant. Again 1 e4 or 1 d4 can reach it.
Strictly speaking this is not opening theory but anticipating middlegame pawn structures. However it does allow you funnel your openings to a few structures and so if you study the structure and you can get it more than half the time you can apply your knowledge and the time invested has good returns.
I haven't really figured an ideal opening for myself. I keep changing things up and testing the waters. The Sicilian for me is more like an opening that I wish I knew better. I'm probably more likely to win with something like modern defense, but I tend to prefer active dynamic open games. I'm likely to castle in the opposite side of my opponent just to up the chances of breaking up the center to target potential imbalances. I would be more than happy to try various openings and figure out with someone what I should probably study specifically.
ps: I had to look up carlsbad and isolani pawn structure. I'm afraid my knowledge about pawn structure is as limited as trying to avoid pawn islands because they can become targets and not getting doubled pawns unless it's convenient for other reasons like opening a column for a rook. I'm not quite consistently getting games that are decided in the very last moments by a slightly stronger pawn structure, but it's good to know. I think the london system, french defense and caro kann tends to lead to carlsbad structure while the more open games like italian/scandinavian might lead to isolani. Are they supposed to be easier to learn because there's less concerns about pawn structure in the end game than something like sicilian ?
In this game: Chess: jovialdick vs blueemu - 200865884 - Chess.com
... we didn't leave known theory until move 28, and we were still in my prep when my opponent resigned.
Being able to have that level of prep for a game and doing anything remotely similar to what you did here would be quite an achievement here. Just looking at the analysis of this line is showing me book moves I wasn't even aware existed. Also, the game seems to flow really well for both sides in terms of piece activity. There's a few inaccuracies, but this feel like a really strong game overall. How do you go about building this kind of prep ? Do you think you can explain how you've build up this level of play ?
Being able to have that level of prep for a game and doing anything remotely similar to what you did here would be quite an achievement here. Just looking at the analysis of this line is showing me book moves I wasn't even aware existed. Also, the game seems to flow really well for both sides in terms of piece activity. There's a few inaccuracies, but this feel like a really strong game overall. How do you go about building this kind of prep ? Do you think you can explain how you've build up this level of play ?
Experience. I've been playing the Najdorf since 1971 (I'm now 65 years old).
And of course, I've suffered my share of disasters with it. But that's OK... you REMEMBER your losses.
If you wanted to read my own comments to the game, you can find them here:
A Heroic Defense in the Sicilian Najdorf - Kids, don't try this at home! - Chess Forums - Chess.com
This is really well annotated and insightful. Thanks for sharing this game.
I understand what you mean, but I feel like the only way to have a clue about intuitively knowing what good moves are in the first place is to learn/memorize what are these moves in the first place.
If I had to single out one chess book that made the biggest impression on me and changed the way that I try to find good moves, it would be Pawn Power in Chess by Hans Kmoch. The book has a big section on various typical Pawn formations in the center... interlocked Pawn chains as in the French, mobile Pawn phalynx, Benoni formations, isolated Pawns in the center, jump formations, and so on... and how each type of central Pawn formation influences your choice of plan and ultimately your choice of candidate moves to examine.
WARNING! - Kmoch does not hold your hand! Pawn Power in Chess is not for the faint of heart... it is a chess book for knuckle-dragging, hairy-backed MEN, not for dilettantes. The topic is dense, and so is the author's writing style. But it is also a work of genius, which will change the way you approach the game... if you have the patience and the persistence to absorb it all.
it's just that trying to reach a carlsbad or an isolani is easier than reaching a typical sicilian structure because mostly you only get this from a sicilian.
so you can try to build around this concept and possibly reach this structure pair with some regularity. of course there are other tries for the opponent to deviate but for the most part that's true of any other attempt to build an opening repertoire anyway.
Correct me if I'm wrong, but are you suggesting that the Sicilian Kan Variation pawn structure is sub optimal and that the Najdorf one is better because it's following a carlsbad pawn structure ? In simple words, I don't understand what's the advantage of having a carlsbad or isolani pawn structure.
Post #36 - That's not a "Najdorf Pawn structure"... it's a Boleslavsky Wall (aka Boleslavsky Hole). It also occurs in the Schveshnikov and the Kalashnikov.
no it's not suboptimal per se, it's less common and only confined to the kan and maybe the english opening leading to a reversed sicilian kan, so you won't get it anywhere else. the najdorf pawn structure with d6 and e5 is considered more dynamic as far as i know. it is true you can probably only reach it via a najdorf or a reversed najdorf but the possibilities for active play are better.
it is the possibility of reaching the pawn structure with other main openings that make it more worthwhile to take it up. that means when you decide to learn a new opening later on you could try to reach a pawn structure you know already from another opening altogether.
with the kan a6 e6 c5 that is severely limited. this means your knowledge of typical plans for both sides will be confined here to this opening and to it's reversed counterpart and nowhere else. at least as far as i am aware you cannot reach the kann pawn structure in any other way.
Well, if you're good at tactics, endgames, and you know 15 moves of theory, then...
... then you wouldn't be rated under 1000
But sure, studying your weakest area is very useful. If you think that's strategy then go for it.