Thresh when built as a laner is a character that relies on souls. Souls as explained in the ability notes are your greatest scaling factor, with them you become an onhit character with some burst and waveclear. But, you're still a support, this means that even if you go badly in lane you can function exactly like a support, hook people, save other with the W and disengage with the E.
So then the playstyle is simple, play safe early game while abusing a bit of your range and then abuse once you get enough attack speed.

As you read this guide you'll realize that it is mainly focused on the build. I don't intend to tell you how to play thresh since i'm not a main, but let me offer you some guidance. CryoBeats, Youtube(https://www.youtube.com/@CryoBeat), is a main thresh and he often plays it top he may not be high elo but he's a good example of how to play the champion.
Other than that I left you some notes on the build be sure to check them.

The thing about this pick other than the limitations you're thinking is that it's inconsistent. Why? Because a soul dropping is random they don't appear each time you kill a certain amount of minions the only ones that are sure are monsters, large minions and champions. So every time a minion dies it's a gamble of 33,3% that it drops a soul.

This is my first guide so please if you find anything wrong tell me. In the future I will drop a few more but for starters I wanted to start with this one.

Bye, enjoy the build.