We don't currently have a way to create custom templates, but there are two things you can do instead. One I've seen is to build the plan you want (name it Template) and then use Copy Plan to create copies of it when you need. And another exciting one is to use Copilot in Planner to effectively build a new plan for you based on a natural language prompt. And yes, premium plans can be exported to Excel (coming soon for basic plans too).