@merber There is a way to preview templates without running an import for each, and it’s arguably the best way to go about it. First, you either set up an import format for each template you want to test, or alternatively, you dedicate an import format for testing purposes (I do this with an import format I named “Testing ground”).
Then, rather than actually importing a literature note, you open the Data explorer provided by Zotero Integration using the command palette. Then, you use the template preview feature of Data explorer by selecting the template name from the dropdown, and then selecting the Zotero item to preview.
This gives you a preview of the resulting literature note, which updates as you make changes to the chosen template.