Instead of Redbubble, which eats a massive chunk of your profits, I'd recommend setting up an Etsy (or similar) and sourcing your own merchandise through suppliers like StickerApp and Vograce. Vograce in particular does a variety of things, and their wholesale prices are very reasonable.
As for how well original comic merch sells, I couldn't tell you. I've had some success selling small quantities of fan-merch, and I'd likely have had more if shipping out of Australia weren't so broken right now. But I've only tried to sell original merch once, and I didn't have the audience to sell a lot of it.
My advice (and this is for Etsy, because I've had way better luck there as compared to anywhere else) would be to have a bit of fan-merch in your shop, from something popular you like, to draw people in. Then, ALSO have your original merch, so anyone who loves your fan-merch, and who goes looking for what else you have will see it, and maybe fall in love with it!