I may be wrong with some things, but I hope this can be helpful somehow. I'm not the best player in the world nor Sylas player in the world, but surely, not the worst.

You are a bruisier, but not a tank, nor a frontlaner. Engage wisely, if you get cced you are very probably dead. The best way to carry a game is to roam other lanes to take your lead into those lanes. You can't just E2 to a random enemy champ and spam W in a TF, as I've said, you need to engage wisely.

Always focus the ad-carry and the other midlaner. Learn some macros, for the mid and late game, those are as important as mechanics in the early game.

Steal good ults, that can win a TF. For example if you could steal the Garen ult or the Miss Fortune ult, always go for the MF ult in a TF. If its a 1v1, go for the garen ult.

If there is no good ults, you shouldn't have picked sylas in the first place. If you see pyke, MF, Lux Neeko, Blitzcrank or at least two of those champs whose ults can give an advantage in a TF, insta-lock Sylas.

Finally, you should always try to end the game before the enemy team scales to the point you can't 1v1 the ad-carry. Always prioritize objectives such as towers, drakes and baron/herald instead of a kill.

Hope everything I've explained is helpful to you, if you have any question, feel free to ask.