Yes, you're right. The ninjette gets a little upset over bumps. A nicely tuned rear shock will help with that a lot without feeling too squishy. Make sure to read up a bit on the basics of how to tune rear suspension for damping and preload (sounds like you already have a good idea
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I know nothing personally about the different brands of shocks. My point of reference is from riding a full suspension mountain bike and adjusting air pressure and damping based on trail conditions and balancing the feedback I get vs the bike bouncing over the trail.