Its not an issue more a Question.

Is there a way to set the custom template i made as default for singleplayer?

Yes, that's how I did it in my own modpack: https://github.com/MelanX/GardenofGlass-QuestbookEdition/blob/master/config/forge-common.toml
This will set the default world type to Skyblock.

so when i want a specific template its skyblockbuilder:custom_skyblock_template right?

Oh... No, sorry. If you have only one template, you just need to set it as in the link.
If you have multiple ones, it depends on your MC version. In 1.16.x, it's just replacing the template.nbt file. In 1.17.1 it should be the first entry in the template config list I guess. I don't remember correctly, but I think your using 1.16.x anyways (because that's what most users do).

now I understand that many thanks for the quick help