Literally every high tier player says that the best way to improve if you're Bronze or Silver is to pick one champion and just keep playing them.
You get much better at the game much more quickly because you actually have the chance to learn about the fundamentals of the game (cs'ing, roaming, objective contesting, teamfighting) when you aren't spending your brainpower tackling what the champion you picked happens to do.
Games in Bronze and Silver are won and lost primarily due to teams making general mistakes rather than champ or team matchups, and you'll reduce your number of mistakes dramatically when you're playing a champion who you don't need to think about very hard to remember what it is they do.