Lets see what I can throw out.

IIRC, and that's no guarantee these days, the XHD springs are around 160# rate. So not radical high, not really low. I doubt they are too high for your vehicle. Remember that if the springs take 160# of pressure to compress one inch and your tires have 35# or air supporting them, that is a huge difference in resistance, especially with a 70 series tire. I wouldn't worry about this aspect.

I wouldn't say a radial wants more or less spring than a bias ply as it all part of the combo. If anything, the construction of a radial may allow a softer rate to get similar performance because of sidewall compliance, but that's tire theory way beyond my pay grade.

Oscillation or porpoising is typically because of match front to rear spring frequency, not rates. But, you say these are relatively minor oscillations of small road irregularities instead of major dips, so I wouldn't say this is the issue.

I might lean towards an issue with high speed resistance of the shock. Shocks control spring travel with high speed valving, like bumps and ruts, and low speed valving, dips on the road or cornering. These are two separate values built in to the shock. Single adjustable shocks are designed to address overall compression or rebound and do not differentiate between high and low speed pistons speeds within the shock. To generate enough high speed resistance might require cranking the shock up so high that the low speed resistance is now producing a rough ride.

Since you have gone to a double adjustable now, I agree that you might want to try softening up the bump to allow it to soak up the hit and firm up the rebound to push it back into the pavement faster.