I would do a couple of things. I would create a brand (somehow related to the type of t-shirt designs you are planning to sell) and buy the corresponding .com domain.
+ be on the lookout for good expired domains on the same topic (clothing, t-shirts, fashion, etc) with good metrics and purchase those. Then either do a redirect or rebuild those sites as PBNs for your main site.
T-shirts niche doesn’t necessaily need to be great at G**gle search results, Pinterest, Facebook, even some hip videos in Tik Tok (depending on your target demo) would be much better investment.
Good luck and if you like my answer, please UPVOTE. Thanks.