I like the way you’re thinking. I could purchase another OEM passenger coil and have both of the long coils with the spacer on top of themFrom your description, I get the sense that you're oversprung in the rear. Meaning at the lower ride height, that too much of the rig weight is transferred to the physical coil springs. The AHC system is not carrying enough load at that point and unable to impart enough damping. Seems to be what you suspect as well? It's much like a stock LX when kneeling, is unable to provide enough damping, as all the weight is dumped on the coil springs in that mode.
If I had to guess, I'm thinking the uprated rear spring is too much spring rate for your relativley stock weight rig? What you probably are looking for is a taller spring with stockish spring rate. Longer free length to get taller ride height. With lower rate to get slinky long travel?
It's a funny balance between spring free length and spring rate. Each can sometimes be traded to get the desired ride height. They are different things though.
I think you might find more success is using the stock spring, 50mm spacers, and the axle mount relocation. Then optionally airbags. I really like bags because they are a magic component to get adjustable spring rate.

