The CH is actually a great thumb rest.
A new handle is last on my list of considerations, but it is (still kinda) on the list.
So far, I haven't had any problems operating the stock CH, and I (almost) always use gloves with the Scorpion, so no grated knuckles yet. Honestly, other than charging the weapon initially, the CH isn't used that much, at least in my experience.
The two most needed upgrades for the Scorpion (imo) are:
1) Trigger. It sucks really bad. Best bang for buck hands down.
2) Safety(s). Especially the one nearest your trigger finger (HBI ak style is AWESOME, jus' sayin').
Grip would probably be my third pick, followed closely by an extended paddle release (I LOVE my GHW paddle release).
Everything else is subjective dependent on personal preference .