If the conifer trees are so prone to fire damage that they require millions of acres of glyphosate to help them recover, maybe it's time to let more fire-hardy species replenish instead.
The area has historically been populated by fire-hardy species in the past. Why not return it to a more natural cycle, especially now that it's being exacerbated by increasingly erratic weather systems?
*Edit: I just checked, and...Oh no... the conifers are the fire-hardy species. They've been in California surviving fire cycles for 50 million years.