What is DG

decomposed granite?

Decomposed granite.

Id plant first then apply DG but I don't think from a horticultural view it makes much diff

I would plant first, being sure to install the plant higher in order to leave space for intended DG depth. Installing afterward would just spread and mix soil onto your fresh granite no matter how careful and tidy you are.

I've used the stabilizer and... Meh. It's just a tacky and softish glue and the manufacturer recommends reapplying yearly. It makes a mess if over sprayed or dripped. I didn't notice any reduction of weeds and it didn't prevent wash during a decent rain event. I applied it to a sloped walkway after tamping to hold granite in place and it wasn't effective. But to be fair, it was practically a flood. Plus, the stabilizer really decreases water permeability, so there was a lot more water shearing along the surface.

I think if you completely saturate the granite at least 2 inches, it might be more effective. But that would be so expensive. I used the large container (3? gallons ~$85) to treat maybe 200 sq ft and it wasn't enough to saturate more than 1/2" at the very most.

When I tried to move the granite back and tamp after the erosion, the residual glue and the chunks made it impossible to tamp neatly and evenly.

So I'm not a huge fan of the stabilizer. It's great in theory, but is lacking in real life practice.