Honestly, I just spent a good bit of time watching educational Diamond/Master streams, listening to their advice and examining why they made certain decisions and their consequences.
Unfortunately, I live at home and can't play frequently (I get ~5 games a week by staying up until about 4am on Friday and Saturday, maybe get additional game(s) if everyone is out of the house). Nonetheless, I just try to apply what I have learned over the years when I do manage to play and am solidly a high Platinum player.
I would recommend working on your fundamentals, such as decision-making and basic mechanics, since those will be the most influential for every game and every champion.
Some items that I generally recommend players work-on:
- Last-hitting minions (to secure gold income)
- Wave management (know how to push, how to freeze, when to push/freeze, when you can safely roam, when it is safe to proxy)
- Rotations (when should you roam, where you should be at certain times)
- Objective prioritization (When certain objectives are safe to take, which are more important at certain times, etc)
- Trading in lane (knowing how to abuse cooldowns, knowing when you can make a favourable trade, knowing how to use minions to your advantage, managing health and mana)
- Item builds (knowing how to adapt to the situation, knowing what advantages items provide in certain situations or match-ups, knowing how to build when ahead or behind [as a team and individually])
- Recognizing when a certain champion doesn't really suit you (i.e. you can't reliably carry with them or you don't have a good win-rate)
- Learn when certain rune/mastery set-ups are beneficial. I mention this because I can see you use Windspeaker's Blessing on Darius and Illaoi and you use Grasp of the Undying on Pantheon; there are more efficient set-ups.
- Learn proper pathing (how to rotate efficiently for farm, what route to take to cut off the enemy escape or intercept them)
- Warding positions for certain situations (i.e. split-pushing, contesting dragon or baron, providing vision when sieging a tower)
- Kiting (as both melee and ranged champions) - Basically, repositioning after each basic attack to create or reduce distance.
If you have any more specific questions or want clarification, feel free to ask.
Edit: As Joe La Frite mentioned, try to learn (in a general sense) how every role players. This will help you predict how certain match-ups play out, how both your enemies and allies will react to certain situations (i.e. Allied Bot tower falls -> Ally ADC Freezes, enemy bot lane and allied support roam mid).
This is especially important if you are a jungler, since you can recognize which matchups would benefit from or require help, when enemies are likely to take certain actions (i.e. ward, push, roam) and how to play out the game.
I main Jungle and Top. As a jungler, I have managed to get so many free kills by simply predicting then the enemy top laner is going to ward the river bush and getting into it about 5 seconds before he tries to ward it. I have gotten more by knowing when the minion wave will push into our territory, leaving them overextended and vulnerable.