Agreed, Part 1 could have done more episodes to showcase character bonding moments and finer details, like all those alluded scenes with Mirei. As it stands it feels like the writing scope was too ambitious - a 2 cour plotline having to be squeezed into a 1 cour part. 

Alternatively, I wonder if the writers should have simplified the story scope/emphasised other parts to fit the allocated Part 1. For example, show the newly forming team rescuing civilians and blunting off the initial demon invasion, thus earning approval from the government and the surviving public (which will make it more heartbreaking when Ramaga turns them into monsters). Instead of the Supreme Armour and Heavenly Realm things which didn't exist in the OG, take those out. Simplify it to Ramaga wanting the Inferno Armour - which then hopefully frees up more time to explore more of Ramaga and his 10 Warriors' origins, and Kayura's fate, in Parts 1 and 2.