Dragon does not spawn when using other end mods such as Stellarity. Summon command also does not work.

Dragon does not spawn when using other end mods such as Stellarity

That datapack is pretty big and does a lot of changes
You should not expect any end-related mod to properly function when using it

Apologies, but SiverDX is right. Stellarity is quite an extensive overhaul, and on top of that it's done via data pack which makes it much more difficult to add compatibility for. I have no plans on adding compat with it at the moment. However, I do want to make my mod as compatible with other mods as possible, so if you have problems with other mods please open a new issue report and let me know!