There are some self-published gems out there, but it's a Russian roulette process finding them.

You are also absolutely correct about self-published authors not necessarily being marketing experts. This self-published author certainly isn't. I did have one relative hit among my self-published titles - Children of the Folded Valley in 2014, which sold over 10,000 copies - but I honestly don't know how I did it, as I barely did any marketing. It seemed like a total fluke. I also think my writing is far better now. If I were writing Children of the Folded Valley today, I'd approach it a little differently. At least one chapter is a bit of an info-dump. Had I been taken on by a mainstream publisher (and I came very close with that title) I perhaps would have had better guidance.

I've since had three novels traditionally published, by a small indie publisher in the US, but with nothing like the same sales. I'm about to try with mainstream major publishers again for the next novel, so here's hoping.

Anyway, all of which is to say I think your gut feeling is spot-on.