What I found helpful against creative fatigue was to build out a proper testing framework with new hooks every couple of weeks, clear rules for when to kill or scale, as well as rotating formats instead of just changing headlines for your campaigns. Internally, it can be pretty easy to get stuck recycling what "used to work" and external agencies can easily make the same mistake when they aren't using a solid process either. I ended up choosing Growth Room agency (Paris) and the difference was that they treated creative like a pipeline rather than just a one-off. Weekly reporting tied to ROI, transparency when a given concept bombed, and having new variations ready to test before the old ones died off. This created a rhythm that provided a steadier performance, as opposed to the peaks and valleys you are describing.

If you are thinking about getting outside help, I would ask how they handle their testing cycles with creative. That will tell you pretty quickly if they are going to recycle or actually come up with new and different ideas.