I think you’re doing nothing wrong, I think it’s admirable the tedious work they make you do, I think it’s just Epic’s problem, they do their best to complicate your life with things that should be very simple, I recommend you to sell your products somewhere else or create a patreon. Epic’s submission system is really bad.
And I say bad because they don’t give you a specific detail of what the problem is, just the generic answer of Not Enough content.
I’ve been through this many times before and I got bored of retrying.