Salem's not that great at planning, if he's honest.
He enjoys having a plan, likes to know exactly what to expect from any given situation. However, he himself is not particularly talented at creating them. He tends to overthink, to try to think of any outcome and what to do if any given thing goes wrong. He makes it way too complicated, to the point where it's far too confusing to follow at all.
This is not a good quality to have, if Salem wants to finally focus on bringing down Catrine once and for all.
All these years, he's just been waiting. He knew that it would be much easier to do this if he was in internal affairs. However, Catrine isn't stupid. Annoying and horrible and the bane of Salem's existence? Sure. However, she does have a brain on her. She couldn't have gotten as far as she has if she didn't. She also has a lot more power than Salem thought, since her and her sister lead Black Fucking Cross.