Have you tried any drill on the flat to improve overall stability? A drill such as catch up would be good for this, holding the stoke in the “set up” position for a couple seconds before taking the next stroke. Although, this removes the fluidity out of the stroke, that pause will improve core stability.
As for paddling in the conditions you describe, time spent in those conditions will lead to the most improvement.
The most important thing is being relaxed. And thinking about at what part of the wave you are putting the paddle in for the catch. You may not be able to keep a consistent stroke, and there will be sections where you put in a few shorter, quicker strokes.
At 6:00 minutes into this video, there’s a good section where the course goes past a cliff section with some rebound. Interesting to see how the top guys handle it. Jeremy cotter just cruises through it.