Custom Template for one Taxonomy Term

That’s easy, instead of trying to change which template gets loaded, load the single-grants.php template, then use get_template_part inside it, and pass different values based on the terms the post has.

Be careful though, single-termname.php is not a good naming scheme, as it could be abused, e.g. you could encounter clashes. It would be better to use a naming scheme such as single-grant-termname.php