There's no point even thinking about going pro before you get to Masters at least, and no point in seriously thinking about it until you get to challenger.

Small champ pool is fine for climbing solo queue. There are plenty of one-trick-ponies even in challenger.

If you were such high rank at other mobas, you should be able to jump right into ranked when you hit 30.

/r/leagueoflegendsmeta or watch pro streams. play the game a lot.

You'll be able to move forward when you know every champ's abilities and every item. If you got to 6.5k in dota you have the skills you need already to analyze lol's builds.

As for how to end a game, typically you want to zone from farm, take tower, roam to get the rest of the outer towers, then either play around neutral objectives, split push, or continue to power down whatever lane is least defended. Once you get inhib you can try to end or you can play it safe and go for baron then take another inhib and end. But this changes so much from game to game that you're better off just learning through experience.