I have read this piece a good dozen times trying to wrap up my thoughts. Here are some conclusions I have come to that aren’t focused on the balancing of abilities.
I originally created Celestialism to be an empathetic option to voidal magic and offer deeply emotional story moments for characters that weren’t necessarily tied to ST driving an event. Your piece does a really great job of capturing this idea within the myriad of spells and abilities. The nuance you are giving to being a Celestialist while still laying everything out in blatant terms is something I only ever hoped to achieve. It really inspires me!
I am not 100% sure where the ST stand on the overall concept of Celestials but I would love to add my two cents. When I created them I did outline that they are Voidal Horrors but with a few alterations. Instead of a Heart of Horror as a core they have a Celestial Heart. The difference is that Horror Hearts feed off of knowledge & life (i.e. they also eat celestials in their metaphysical forms, driving their motivation to seek the help of Celestialists) whilst Celestial Hearts feed off of knowledge and with the help of Celestialists, fonts of mana (or some equivalent). The other difference I had was as you delve deeper and deeper into the Void they become less common and the closer to draw to the Veil, and the mortal realm, the more common they become. Because of this I tied them directly to the Veil… which leads me to my only actual idea for your lore;
Celestial Gardens are antithetical to voidal tears, hearths, and hollows. Where those 3 thrive off of pure chaos a Garden has a purely symbiotic relationship with the mundane world. So upon reaching the upper tiers of Celestial Gardens they should act as countermeasures to Tears, hollows, etc… Making it harder to open them in the vicinity and also acting as a way to stabilize afflicted areas. Have a tear or even a hollow? A Celestialist may come to plant a cosmic seed allowing the area to stabilize and establishing order over a set period of time. Gardens bolster the Veil directly around them, becoming a hard spot in the thin crust that helps reduce/prevent voidal incursions.
Besides that should this come to be accepted I would love to work with you on developing supporting lore around the magic. Where did it go, why is it back? What happened in the time it was gone? Etc…. I think a magic like this benefits from having other pieces to latch onto that aren’t just “cthulu void tentacles bad. Void mage bad.” Thoughts?