I really like the work you've done on these. However, I do have one minor critique. In "Godzilla in the mountains" and "Kaijufornia Spring," seeing the kaiju interact with/overlap objects in both the foreground and the background makes it difficult to establish their scale. In "Godzilla in the mountains," the monster's tail passes in front of a fallen tree in the foreground, while one of his hands appears to be resting on a rocky outcropping a significant distance away in the background. The former implies this Godzilla is actually fairly small (for a kaiju), while the latter implies he is absolutely massive. Similarly, in "Kaijufornia Spring" Leatherback's right hand (resting on the hill in the foreground) is about the same size as his left hand (wrapped around the thin tree in the background); a cow in the same plane as his foreground hand is nearly the same size as that hand, while a cow in the same plane as his background hand is barely the size of one of his fingernails. I feel that, by paying a little bit more attention to the perspective in these scenes, you'll be better able to capture the grandeur of these creatures as a part of the landscape.
Again, excellent work, and I hope to see more art in this series.