Also this post will assist. Between those two posts, they largely define which legacy drones can be used in A1/A2/A3, and what the limits of those three sub-categories are.
Basically: if you want to operate outside those limits (closer to people, above 400-ft, BVLOS etc), then your next step is a GVC/OA. If you can do everything you want within those limits, then you are fine with no qualification (A1 or A3), or with the A2 CofC (A2).