Right now there's no single way to create a task from a template that both enforces required Custom Fields and brings in the template's attached Doc. Applying a template to an existing task includes the Doc but skips the required field check. Loading a template in the create-task window enforces required fields but drops the Doc. Getting both means building an Automation as a workaround, which shouldn't be necessary for something this basic. Please make templates enforce required fields and carry over all template content (including Docs), no matter how they're applied.