You've already got some great options in mind, like screen printing and custom t-shirt printing. As for more specific keywords, targeting school shirts, church shirts, plumbing shirts, etc., can help you reach a more targeted audience.
To determine which keywords are most popular in your city, you could use tools like Google Trends or SEMrush.
If you're still struggling to get traffic to your site, you can buy seo traffic. It can quickly and effectively boost your site's visibility and get more eyes on your products.