I'm an ops coordinator, not a designer, but somehow I became the person every team dumps their deck on. Fully self-taught, mostly by making ugly slides in front of people and slowly fixing them.
The stuff that actually moved my decks from "clearly made by ops" to "oh, that looks decent":
Pick one left margin and start every title, bullet, and image on it. Boring, and the single biggest upgrade.
Cut the deck to one point per slide. If I'm cramming, I split it. Nobody pays me by the slide.
Learn exactly one nice thing well instead of ten badly. For me that's the morph transition. Used once or twice in a deck to move between a big picture and a detail, it looks intentional. Used on every slide, it looks like a screensaver.
Make the title state the takeaway, not the topic. "Q3 numbers" became "Q3 came in over plan," and suddenly people follow along.
None of this is real design training, it's just the four things that stopped my team decks looking amateur. For the self-taught crowd, what's the one trick that made the biggest jump for you?