Painted render

After the best way on adhering to painted (solid) sand and cement render.
As long as it’s solid as you said I’d use a r7 primer k-rend hpx or equivalent mesh and anchor fixings. Obviously never ideal to go over painted render.... maybe score up a bit with grinder.
Agree with Mr. Riddel, score it up.
Small grinder with diamond cup disc will make short work of it.