I feel like grass is already a really good matchup at the moment. Only Gyarados and Sandslash have a weakness to grass, while Crobat and Charizard have a 4x resist, and Nidoking hits it for super effective poison damage.
The main issue with Scyther, and the only reason I didn't include it, is that 4x weakness to rock.
I ran with a USUM Mono-Bug team and loved it to pieces in competitive.
You do not know pain until you send out Volcarona or Mega-Pinsir and they immediately lose half their health. You already have one 'mon like that in Charizard, and the other flying 'mons have issues switching into rocks already. (Also, Stone Edge is a super common coverage move and it's just bad feels when you run into it.)
This slot is basically Crobat vs Scyther, and I think Crobat does a lot more stuff that the team could really need. Fast Taunts, Defogs to remove rocks, and it can Roost to remove Stealth Rock chip. Can also pivot quite well with U-Turn to scout and slip in your best sweeper, though that's either in the Brave Bird or Taunt slot which limits utility.
So the water weakness is partially mitigated by Charizard's Drought, but anyone setting Rain Dance is... going to give you a bad time. A really bad time.
Also, if you didn't want to cheat, I feel like you could still do well by using Dugtrio in the Steelix slot. It is, to be clear, not that bulky, and doesn't add any new resistances. By all rights, it should be bad.
But. Arena trap. Memento. Final Gambit. Reversal/Focus Sash. Dugtrio's going down to most anything, so who cares what type it is? It's basically a typeless utility move user, which the team could really use.