Okay, to be blatantly honest, all these designs are okay-ish. It still looks like a low-tier T-shirt. The design could be WAY better, like you could write something on the sleeves, and make the T-shirt look more interesting enough for a stranger to look at the T-shirt and go like, "Wow, I like this T-shirt". The current humor or"memes" used on these designs aren't as good. Right, the t-shirt looks like someone took a plain T-shirt and added some images from google on the t-shirt, and that's it.
Also, have you considered selling not just T-shirts, but hoodies, tank tops, crop tops, sweatpants etc?
Not to be harsh but i do fully agree into your words when it comes to design which isnt really that great or not catchy i say.
I do rather prefer this one and do fit out my taste even if its just a plain BTC logo.
Source:https://www.pinterest.ph/pin/86835099050228064/
But actually having this kind of start-up isnt bad at all, it do just need some design improvement.