List of openings that you can quickly pick up (TL;DR)
- The Russian (Petroff) defense (much better choice for keeping it simple than 2. ..Nc6 due to tons of theory and especially great because it directly follows the classical opening principles and gives Black a very healthy position. It is far not as boring/drawish as people think on amateur level.)
- System openings (London system, Colle system, King's Indian attack, Hippopotamus, ...) [Note: easy to pick up, but I don't recommend them]
- The Caro-Kann defense
- The Chebanenko-Slav
- The Queen's Gambit Declined (QGD)
- The Accelerated Queen's Indian defense (hypermodern, has some similarities to the QGD and the Nimzo-Indian. Objectively problematic in lines where White plays f3!!)
- Anything that IM Sielecki recommends in his Keep it Simple series (Rossolimo / Moscow against the Sicilian, QGD,...) [Note: He also recommends some openings that can become theory-heavy, so you should follow his analysis to cut theory, e.g. play the Ruy Lopez with d3]
- Sidelines (e.g. Grand Prix attack against the Sicilian, Vienna opening/gambit, (Hyper-)Accelerated Dragon if you desperately want to play the Sicilian, etc.) There is also a Chessable series on the Sidelines approach for White by IM Banzea [Note: I would always prefer less popular variations within mainlines to sidelines when trying to reduce theory]
- The Scotch Game (according to Chessmood, but in my personal experience it's a little "computerish" if you really want to squeeze out an opening advantage)
Detailed answer
First of all, before concerning yourself with concrete opening theory, you should have a very firm understanding of opening principles (and follow them). This already brings you very far. Theory moves are often moves that satisfy the most (opening) principles at once.
Before memorizing lines of an opening, it is also important to understand the fundamental ideas of that opening first. For this, you'll need to find instruction, e.g. from a book or video course, where a strong player explains the purpose of the moves you do. This also helps tremendously in memorizing them.
Now, to keep the workload of learning concrete theory of an opening low, you're looking for openings that satisfy these requirements:
- not too many choices for the opponent
- not too many forcing and long sharp variations (i.e. a move-order mistake shouldn't knock you out immediately, cutting the need for heavy memorization)
- overarching themes / structures / ideas across different variations (so that you get a feeling for and understanding of the opening and related structures with clear plans, ideas, pawn breaks etc.)
- Little diffusion, i.e. small changes in the position shouldn't have a large impact on what you need to play.
- objectively good (e.g. doesn't give Black more than -0.3)
Naturally, more strategic openings with fewer forcing options lend themselves to these criteria.
That means that mainline Sicilians are out of the question, as well as most hypermodern openings because the opponent has a lot of challenging options and you need to know a lot of concrete lines in order to survive.
I wouldn't sacrifice too much objective value, but you may want to look into sidelines / sidelines within mainlines because they have less theory.
For example, instead of playing 1. d4 and following up with the absolute mainline 2. c4, you may play the London system.
However, 1. e4 openings have a lot of advantages over 1. d4 openings for beginners (move orders, transpositions, changing of lines against Black replies), so I'd rather recommend this, even though you may need to learn a little more: it will help you to become a better chess player and have more fun.
I would generally not recommend Hypermodern openings, such as the King's Indian defense that you play. White has a huge variety of dangerous options (1), Black needs a lot of chess understanding and feeling for how to play closed positions (where even GMs say it's difficult and that you need years of experience to do it well).
There is an incredible amount of theory to the King's Indian defense and many GM's stop playing it after some time.
The king's Indian was also the first opening that I studied, and while I grasped some fundamental ideas quickly (e.g. e5, f5, c5 pawn breaks and pawn storms, a5 to develop the knight via a6 etc.) and my results weren't even too bad, I soon realized that it isn't a good opening to start out with and started to play something much better:
It follows the classical opening principles but is a fairly rare continuation within the Slav defense, which is one of the most frequent replies to 1. d4.
GM Colovic explained all the ideas and plans very clearly, and you are aiming for a very simple setup (basically a "reversed London system").
The Chebanenko is the "purest" Slav in the sense that you'll almost always achieve that setup.
And you can play that setup against nearly everything that White can throw at you except 1. e4.
Chessable Quickstarter chapters
I can recommend Chessable very much for your situation because you get
- Grandmaster explanations and therefore a better understanding of the opening
- A Quickstarter chapter, where only the most important lines are selected that you need to know before you can start playing that opening. There are some openings, where you only really need as little as 13 lines to start playing them!
- a move trainer with spaced repetition technology, so that your time and memory retention are maximized
Especially, I recommend you strongly to look at IM Sielecki's Keep it Simple series (e.g. Keep it simple for Black, Keep it Simple 1.e4 v2, etc.) where he provides you with complete opening repertoires that satisfy the requirements I outlined above.
Each of these courses comes with a Quickstarter chapter, and his choices for example the Caro-Kann defense mean, that you only need to know ~13 variations to play it! This is probably one of the best and quickest openings to pick up!
You can also consider IM Andras Toth's Beginner's repertoire series.
Before you buy any course, you can check CHessable's free Short&Sweet version to see whether you like the variations and to try them out in some online games.
[I am not affiliated with Chessable in any way other than being a customer.]