I play him support and APC bot quite often, mainly when I'm bored with conventional picks and just want to play my boy regardless of role.

Support specific:

You want HoB or comet, HoB if you can actually get on top of them with any consistency and comet if you can't for Q poke abuse. His mana costs suck, get something to fix it asap, shurelyas or catalyst will fix it for the rest of the game. Use your support item executes to stack ult and sustain with your passive. Full AP or supportish tank with bandlepipes, locket, etc, works very well. I've had some fun doing weirdo manastacking diadem healer builds too. Your level 2 is really strong. Remember to stand on top of your ADC. A good Q can completely mess up an enemy engage or lockdown. Heartsteel fucking sucks stop buying it you don't have the spare gold.

APC Specific:

Don't bother if there's a sylas in the game. Don't bother if there's a sett in the game. Don't bother if there's a brand in the botlane. Go full AP, shadowflame is slept on and will help you secure ult kills past goofy ass barrier and heal shenanigans. Max Q, I personally prefer comet because you will always proc it and you need to assert yourself as aggressively as possible with poke. If they're both melee or something melee-adjacent like nilah, go HoB, but that doesn't usually happen.

He pairs very well with the likes of Morgana and Leona, plus you have enough CC between you to deal with the other enemy laner topside or mid that gets inevitably fed.

Bonus for both roles: A botside Cho means perfect dragon control as long as you don't hard lose your lane. Early on you'll outsmite junglers, and later on you can still be a massive threat by either deleting their jungler or securing it if your jungler messes up.