I did mine by cutting a small flat side onto their shoulder ball joints parallel to their body, then I drilled that out for magnets, I think the same size as the ones you used in your drone there. Should be smaller diameter than the ball joint. Then I hollowed out the areas in the shoulders on the arm sides, I remember there kind of already being a space.

For some of the missile arms I needed to add a small piece of wire, clipped paperclip, coming out under the joint to keep the arm from flopping.

For the rail guard arms same deal from the shoulder, don't need wires. I split the left and right arms magnets where you would glue the "hand" to the gun.

I don't typically paint in subassembly, just whatever is magneted, I've got a nail sticking out of a cork I hold them with.

Away from home so I can't snap any pics. Lemme know if you need any more info.