Thank you! Taking out the quotes worked. Thought for sure I had tried that but but obviously not since this fixed it right away.
My template part filename DID need to have the first letter capitalized. I had made multiple versions (test.php, Test.php, TEST.php) and it uses the one that matches the output of single_cat_title() exactly.
All my category names are single words so hopefully I won’t have any further issues.
Thanks again!