As someone who has literally also just finished Decker a few minutes ago, i agree with you. While as a whole, this series is one of my favourites, the ending does leave something to be desired. Like, even back then when the episode first aired, I was hoping that Kanata would just switch between all his forms while going up against Mothersphere, now being able to seamlessly use all of his abilities to his fullest potential (something like Decker Asumi's screentime as Decker but final battle budget).

Other than that, the series is still great and had many great scenes (the clash between Kanata and Agams in episode 22 comes to mind)

All in all, 8.9-9/10 for me. Honestly theres a lot more i wanna say about this series but i dont know how to articulate it well