Playing one champ is good for learning quickly if you go the right way about it. The point is to not have to think about your champ so much so you can spend more attention / brain power on other things. Like watching for times you could have played better so you can give yourself a mental slap right away so you can start moving to better play immediately. And watching for small mistakes your opponents makes so you can take advantage of them.
A simple champion is good for learning as there are less things you can do, thus making it easier to work out what your best choice was if you review the game. Kind of like how it is easier to recognize where you went wrong in tic-tac-toe than it is in chess.
Getting good at playing the later stages of the game where how your team mates play has more impact on what you can do is hard at lower ranks because they are more often not playing correctly. Getting good at the earlier portions of the game where you interact with them less is therefor more productive. If you nearly always have a big lead from early game, then you only need to learn how to play with a lead.
The steps are: get good at getting a lead early -> get good at using lead mid to get a good overall situation for your team -> get good at not throwing once the game state is favorable for your team.
Playing out losing games is not productive for getting better compared to playing more early games, so if things are bad do vote yes if a surrender vote comes up.
As to what you can learn to help with mid/late game, you can:
- Try keeping track of enemy summoner spell / ult cooldowns so you can make good choices relative to that information.
- Learn how long is takes champs to travel between different parts of the map, so you can estimate where enemies could be even if you lack vision of them.
- Learn correctly estimate how long it takes to do baron/dragon with different combinations of champions with various items and at different points of game time.
Stuff like that, so you have a better basis for the choices you make. But knowing such things only matters if you have the level/items to make your actions matter more than the actions of others.
