I mained Thresh in season 4 so here's my opinion.
-IMO relic shield is a much better pick over the coin. It makes you more tanky in lane and helps sustain both you and the ADC.
-You need to be a little more aggressive in lane. Use your E when it stacks up full to poke them in lane. Obviously don't do it where they can poke you down for half of your health, but when you ez walked up to get a cs walk up to him and hit him. That one hit could make a difference in a fight.
-As mentioned, quick cast. It helps you aim better and can shave a millisecond off your hook and that time could be what you need to land the hook.
-Buy a pink on your first back when you have the gold. Use this to ward tri bush so your jungler can be assured there isn't a ward there. Of course this depends on what side of the map you spawn on. But buy one either way and use it to provide extra vision for your team.
-You need to ward more early game. You tend to only place 1 or 2 wards in the early game. At times when you had sightstone your dragon wasn't even warded. Also, when they don't have a gap closing jungler that can go over a wall you don't need to ward the bush right next to bot lane. You just need tri bush and dragon warded cause then you can see them coming down river much earlier. While if you only have the bush near bot lane warded and your pushed all the way to their turret it could be too late to get away.
-Work on your flay mechanic. I noticed sometimes you would move your character to flay them when you could have just flayed them right away and your ADC could have taken less damage.
- Sometimes it is better to flay and then hook, rather than hook and flay. This slows them and makes it much easier to land the hook.