I sell merch, mostly prints, stickers, badges, original paintings, keychains and bookmarks. I have an online store with my colleague redartlilas but I would echo what others have said in that selling online is not a good way to make money unless you have a following. I'd say a good 80-90% of my merch sales happen in person when I'm tabling at conventions, and I'm lucky enough to have had repeat customers at conventions who recognise me and are really sweet, but they don't buy online, it's the convention experience and being able to talk with the artist that makes them come back. People tend to buy my stuff because it's pretty, not because they know the characters though, so I really work to make my illustrations have a vibe.
Selling stuff online is really difficult because of the lack of human connexion and visibility. You can't as easily stand out because there's the whole world as a choice and there isn't that time limit on "if I don't get this now, it's going to be more expensive because of shipping costs". Plus people can't just walk by and see you're having an interesting conversation with another customer or doing a demo of how you draw.
I don't use print on demand sites because I like to have full control of production and be able to choose someone else to make my merch if I'm not happy with the quality, and I also have control of my profit margin because where the money goes is 100% transparent on my end.
I'm the opposite of you I'd much rather sacrifice some convenience to make sure I have control of what I'm selling and if there's a problem I can take responsability and people can talk to me directly. It's also really easy for newbie merch sellers to get scammed on those sites because they usually don't know what typical production cost for different products is so it's difficult for them to pick up on something fishy with how much they're being paid (like if the payout for a $10 print and a $10 shirt is the same, big red flag, prints on paper as a rule cost a fraction of what it costs to make a shirt).
Basically if you want to try earning any meaningful amount of money from selling merch I'd recommend looking for stuff like small anime/comic conventions/creator markets in your area. It's not an option everywhere but depending on where you live there could be one right around the corner, and it can also get more attention on your e-shop, even if you don't make any sales from the people who visit from the convention, it helps boost you in internet searches.
I'd also generally recommend doing some market research into pricing (to see if the print on demand websites you use are overcharging you or the customer) and trying to work out what your "brand identity" is going to be so that people will more easily recognise you. For example I use a lot of black and gold, flowers and bunnies in my personal branding to go with my prices that are generally on the higher end and my illustrations that generally include water, wolves, rabbits and/or flowers. You basically have to become your own PR team when trying to sell things online. It's a lot of work.
If anyone wants to check out our merch store it's here:
It's shared so we combined our branding (theirs being lighter and manga-adjacent).