What causes a wing to snap roll on me when I pull too much elevator?
Elevator deflection and forward speed are high enough as to pitch the nose up around the CG beyong the critical AOA of the wing's airfoil respect to the incoming air stream.
Too much "vacumm" is produced on the top of the wing, air stream deattach from the wing's skin, the wing begins a stall inwards from near the more loaded tip.

I see this on nearly every plane I have. Some are better than others, some are really nasty.
Type of airfoil determines how abrupt the wing goes from lifting to stalling.
Wing planform and twist (washout or washing) determine how loaded with "vacumm" the top area close to the tips is.

I'm flying an electric 300 and it snaps on me really easily and it's making it so I can't do things I want to do.

Do I need to :
1) change CG
Only if the CG is too advanced (the wing "feels" more total weight) or too retarded (the elevator has not enough authority for strong pitch up even with extreme deflection).

2) Add flapperon mixing?
Not sure about it; it may help increasing the CL of the airfoil, but it reduces the critical AOA at the same time.

3) reflex?
Not sure about it; it may help increasing the the critical AOA, but it reduces the CL of the airfoil at the same time.

what else should I look at.
Put that airplane on a diet; weight is not your friend.
Reduce elevator throw to the minimum to achieve your maneuvers with no snaps (experiment high enough to recover).
Use low rates for high speeds and high rates for low speeds and 3D type of flight.