Your brand name or logo are eligible to be trademarked. But that will not protect the individual designs, only the name or logo that you submit for registration. The value of this is that no one else will be able to produce or distribute similar products under the same brand name (as long as you enforce it). Trademarks cost $275-325 if you submit the application yourself. It will cost more if you use an attorney or online legal service.
If you want to protect the individual designs, you can submit them for copyright. You can either submit them separately for $35 each, or submit them as a book and pay one $35 fee for all designs. Designs are technically copyrighted once they are in fixed form (even without formal registration), so it's up to you if you want to submit them or not. Copyrighting the designs only protects the exact design, not the phrase within the design. Someone else can still use the same phrase as long as they design it differently.
To protect a phrase, you would need to trademark it. But it is considered very difficult to trademark a phrase for use on t-shirts unless the phrase is specifically used as part of the branding or marketing. A phrase that is only used within one or two designs will probably not be approved.
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