Finally realized I was painting trim wrong after 6 houses

So I've painted every room in my current place twice since 2021, and the trim always looked kinda bad no matter how careful I was. Last week my neighbor came over to borrow a ladder and saw me using a brush straight from the can. He asked why I wasn't backbrushing with a mini roller after laying it on. I had zero idea that was a thing. Turns out I was leaving brush strokes and heavy edges on every single baseboard and window frame for years. Tried his method on one door casing with a $6 roller from Home Depot and it looked like a pro job in under 10 minutes. The whole time I blamed the cheap paint, but it was just my technique. Has anyone else had that forehead slap moment where a basic step you skipped was why the finish looked off? What should I fix next, ceiling paint or cabinet doors?