I don't have any personal experience with quality, but I hear it's good. As well if you're not happy with the quality, you can get a refund quite easily. So it's kind of risk free.
I just finished uploading my designs yesterday, so I haven't sold anything (yet :D). I think you have to do pretty much all the marketing when you're a new user and later if they like your work and you already have some following, they can feature you on the front page for example or on their instagram etc. The beginning is the hardest!!
Never heard of Printful. It looks really nice on your website! Is Printful like S6 and RedBubble that they do all the work for you and you get a small commission? How much traffic does your website get normally per month and where does it mostly come from?
Thanks a lot for sharing your experience in making and selling your products, I'm happy to connect with someone who is on a similar path as I am 