I've gone both the indie route and the traditional publisher route and I'm honestly not sure the publisher is worth the other tradeoffs. TBH, the biggest value of the publisher was that it still carries a certain (not entirely warranted) gravitas.
(I'd argue that if you're doing a book by yourself without a co-author, you pretty much have to hire at least a copy editor but that's pretty inexpensive.)
I don't doubt that even a few hundred dollars may exceed revenue for a lot of self-published works. But I'd argue that someone needs to do a careful read-through of a book. You simply don't see mistakes after a while and most friends just aren't going to do a careful enough read through for free even if they're otherwise qualified.
You paid low-to-below market rate to an ex-intern early stage career and I compared to a 20+ year proofreader in academic for ESL. Probably fishes to bicycles. I think you got what you needed which was a light touch confidence spellchecker.
A new to writing solo author, bursting with ten years of bildungsroman, who got Scriviner and has dumped two thousand disjoint paragraphs of prose into word...
I bought and read a few self-published books on Amazon. The lesson I learned was that you should never, under any circumstances whatever, pay for a self-published book. The average quality of writing is atrocious.
Publishers may not do much direct improvement of your writing, but even if they perform a purely censorious function of printing good writing and refusing to print bad writing, they're adding a lot of value.
(And my understanding is that they do in fact work with you to improve your writing, but I have no experience in the field.)
Of course, historically an 90% commission on a print book would cover expenses like editing, printing, and marketing which Amazon doesn't provide; and the risk/cost of unsold copies, which ebooks don't have. Still, if you think your publisher wasn't putting much effort into the marketing anyway....