Do you use Amazon merch or do you use another provider (printify…) or something else and connect to your Amazon account ? Any recommendations ? 
I am doing that using Printify and also selling custom designs for Tshirts. It is a real pain. ETSY on the other hand is very better and flexible then Amazon POD. Etsy has a larger market in such items and niches. In this group you are only going to find people who have watched ten videos for private label, ten videos for wholesale and present themselves as specialists. Where as they don’t even have any practical and useful information at all.