I would like to be able to set the number of stars for all users. Unfortunately, the /sga:admin command for this only changes it for the user of the command. So I have to get everyone to run it in order for them to see all of my stars and constellations.

you could use https://modrinth.com/mod/client-execution and a datapack

i think setting the starcount does reset star names etc, so youd want to only dp it on first join, eg by giving everyone a tag and then only running it for people without the tag