Rose here from Shopify. Thanks for reaching out!
I would love to provide some feedback on your store. At first glance, it's clear that you put a lot of effort into your business. The overall brand is effective and your products are well presented. Here are a few areas of improvement I can suggest:
In your main menu, I would suggest adding your About Us and Contact page. I can see that you have these listed in your footer - which works great! However, I suggest adding them to the main menu as well in order to improve your SEO rankings and build trust with your customers. The main menu is the overview of your store and business. Easy access to learn more about your business and to contact you will increase the likelihood of conversion.
Relationship between Slideshow Text and Image
I recommend adjusting the image layout with the text on your main slideshow. Right now, the text is hard to read. You can try editing your image outside of Shopify so that the center point of the image is higher. That way, the text can fall along the upper body of the models and the text will not be on top of the content heavy part of their t-shirts.
Above the Instagram feed, you have a quote. The spelling should be "IF YOU'RE NOT THE FIRST, YOU'RE LAST".
Collection Product Display
On your collection pages, I can see there's mostly 3 products per collection. Since this is the case, I would suggest changing the theme setting so that it shows 3 products per row instead of 4. This will remove the empty space at the far right:
You can edit this setting under Online Store > Themes > Customize. From here, you can locate the "Select page" drop down menu and select the Collection pages. From there, you can click on Collection pages on the far left menu and then edit the Products per row toggle to 3.
Lastly, I would suggest adding a product review application to your product page. This will give your customers an overview of the quality of your products with real customer reviews and feedback.
I hope that helps! If you have any other questions, please let me know.