Here we go.
I filtered your ranked games for the games i looked at. From your match history i guess your main role is support?
First thing i see which you always don't buy is a sweeper (red trinket) after you got your sightstone. Think about the fact why YOU buy a sightstone. Because mapawarness is powerful. Get a sweeper and clean as much wards as you can, this takes power from your enemys.
The even better item is a pink ward. i looked 5 games up and you bought in 2 games one pink each. Think about: You get a ward, you clean of an enemy ward and this only coasts 75 Gold. Before green wards were taken off the game everyone bought them for 75 gold. Invest it in pinks now, they are more powerful and worth.
Overall i would say you use wards not often enough. in a 45 minute Thresh game you placed 18 wards. That's 1 ward every 2 minute and 30 seconds.
You always get ignite as a supporter, sometimes its better to go with exhaust to avoid burst damage. I am not talking about the early game where you lane with your adc. I talk about a matchup where you got a yasuo or zed in the game which will do infinity burst damage in the late game. If nobody in your team will go exhaust then you should do it.
If you see the higher advantage in a botlane matchup for the enemy then you should go exhaust also. Instead of a soraka match up, here you can go for your ignite to win the lane.
Why do you always buy the blue upgrade on the shoes? Gives you 20 % movement speed after flash for 1 second. Better go for the yellow one. 10 % movement speed for your nearby approaching allies.
A good thing is you play on bot nearly 95 % thresh and blitzcrank. You get a feeling for the hooks and your plays. What i dont like is the fact you play roulette in the jungle. Tahm, Jax, Master Yi, Trundle, Udyr, Shaco ... it's too much on a champ pool.
I try to concentrate on 2 champs for 2 lanes. You main one of them and go for the second if its banned or picked. I know its boring over time but its the way how you will perfect your gameplay overtime with this champ.
Since i am not a player which goes for supp often i can't help you with your item builds or the match ups as i would like to.
Hope this will do it for the moment.