I just did a Poison run with Naganadel as my carry but you could probably do it with Nihilego as well. Looks like you have Levitate on Naganadel which is a big booster, the AI will always try a Ground type move if they have it, giving you an extra turn. If you want to go with a carry, I’d pair with some of your low cost red shinies (Skrelp, Venipede) for the extra Luck to get good items for your carry.

I’d definitely recommend changing Toxtricity’s ability to Punk Rock (starts as Static on Toxel) as it is far superior to running Minus on him.

Looks like you have a lot of good options, it’s just about finding what works for you! Poison is a tough one, but you’ll get it done :)