It's not about knowing the techniques, it's about doing them WELL. Think about L-cancelling, cancel half your aerial lag by pressing L/R/Z with the right timing. But what timing? The timing can change depending on your fast-fall speed, your aerial spacing, whether you hit your opponent or hit their shield or just hit empty air. Practicing good L-cancelling means being aware of all of this and also acting as fast as you can out of your L-cancel (otherwise, they'd be no point). Good L-cancelling can be the difference between amazing shield pressure and getting shield-grabbed like a moron.
Dash-dances = not implementing more, but implementing them well. Vary dash-dance lengths so you can keep a good distance to keep some pressure on your opponent, but still have enough distance to bait them in and weave back in for a hit/grab. You can dash back and forth all you want, but if you're not cognizant of the reasons for why you're dash-dancing or for the subtle spacings of it, your dash-dances will be pretty much useless.
If you have good combos but they aren't working against people, they're not good combos. Or you are currently not good at converting into meaty combos off good neutral hits. Nair -> grab, knee -> grab, experiment, because if you can't land the combos you practice, they're useless.
If you think you're being too offensive, then stop being offensive lol. Don't approach, bait them in, punish their approach. Good offense is not about running in and getting wailed on, it's about mixing it up so your opponent can't read your approach and spacing yourself so that your approach won't get easily punished. This means not shffling nair straight into a dash-dance grab (if you get hit by this, it means you need to space closer and overshoot your aerial more to run through the dash-dance).
Also, new to competitive smash is about as good as new to smash to me. Tons of players new to the community have played and enjoyed smash casually for many years. That doesn't mean that they don't get absolutely whomped when they play someone who's been in the competitive scene for even just a year or two. Just wanted to say from someone who knows, don't expect your familiarity with the series outside of the competitive scene to help you at all, because it really won't.