Sounds as if you have a vision for the space you want to work in, and some decisions to make. I would suggest that you get out the old spreadsheet and do some forecasts to work out the best way for you to work.
Look at the buy price, any costs you will have, the sell price and what that means for your margin. Then compare the two options and see which makes better sense.
A lot of people I know source tshirts from AS Colour and then customise. I don’t know a lot about that sector but might be a good place to start.
Have fun! Vicky