I WILL NEVER GET SICK OF THIS 😤

So I'd say the single easiest way to get better quality finishes is a quick dry topcoat that you put on before the polish is even touch-dry. Personally I try and aim for just try enough that it won't stain my brush. This, at least in the top coats I've used, actively 'melts' the base coat (which is also why I'll do a lot of designs OVER a thin layer of topcoat, because if I did it on the base layer all together it'd probably end up bleeding).

Also most decent nail polishes are designed to self-level. So in a way, speed is more important that meticulousness. You don't wanna race, but a few confident downward strokes will do you better than carefully trying to get slow, perfect coverage. If you miss a little edge or weren't close enough to your cutiles, you can fix that with a second coat focusing on those areas aver the bottom layer dries enough to not be disturbed by the brush.

Another little this is angle. The more parallel the brush is the more you can gently drag the polish along the nail with minimal pressure. You probably don't wanna be perfectly parallel cause you want the brush to fan a little, but play around with shallow angles.

Good nail polish goes a long way. Not that you have to go crazy but my OPI and Essie polishes behave way better than my budget polishes, unsurprisingly. Another difference is brushes. A lot of cheap brands have very thin brushes that don't fan great. Better brands have brushes that will fan out to a wide area with a little pressure. Smaller strokes means more fussing, which often means more streaks. I haven't done this yet, but apparently you can buy replacement brushes? So some good ones migh make some cheaper polishes actually go on nicer.

So, in order of important/return on investment from most to least TL;DR
* Good Fast Dry Top Coat (I like Seche Vite)
* Preferably applied when the polish is still a little damp
* Fewer, more confident strokes and if necessary, more coats.
* Better brands
* Get better b rushes
* Better brushes

Lemme know if any of these actually help! I know what works for me, but I don't know how transferred my experiences are.