The *most* important thing is to get a good coach/mentor. As you can see, DerEchte needed Bogeyhunter. I was lucky to have coaches and mentors in my table tennis club who never charged me a lot and often nothing at all. In fact, the player I play the most like now is a player that never charged me for a lesson, but whose serve and thirdball style is what I deploy. Just about everyone who improves as an adult has someone who cares about their game to help them. If you find a high level coach or player who invests in helping you improve and you take what he says to heart, it is almost always worth 200 points because they can see what causes opponents at your level problems and teach you how to do it. It doesn't mean you can beat everyone, but you will be able to beat quite a few players by exploiting the typical problems at that level.
They will take you through basic drills, but they will also help you establish point winning patterns and tactics. You have to know how to serve to get weak returns, and then you have to practice putting away the returns you get that are weak. The serve and third ball practice is probably the biggest thing you have to establish and the thing you have the most control over. Rally strokes are important but highly overrated, because the serve and return always comes first, you rally if you are forced to.
Then if you are lucky and have tons of time, you can work on serve return. Serve return strokes are the most unique strokes in table tennis after they serve, they are the only strokes you play vs a ball that bounces twice on the table, so you need to treat them appropriately. Also the opponent has more control over the spin they place on serve, so if you know how to do something to the serve that neutralizes that or which puts the opponent under some pressure, it helps your results a lot.
IF with the help of a coach, you build a lot of your game around serve and serve return, your game improves fast, of course there will be holes, but those will depend on you and how much time you have. But the biggest return on invested time is always in serve and serve return because you are doing them on 50% of the points and 100% of the match, while rally, you may never see.