Meanwhile, while Amazon apparently no longer lets me get checks for book royalties (again, without notice to me), Amazon Associates still lets me choose checks as a payment option.

Substack does have the virtue of being free to use AND not having any advertising, so there's no advertising to buy. As we've discussed before, it also can in a pinch replace the author's website and newsletter provider. Web presence, email, and a fairly large and receptive audience--all for free. It may not work for everyone, but there isn't a huge bar for entry. I guess you do need decent internet, but otherwise, there's a lot of free stuff there.

My take is that if you have access to Word in any shape or form, and you're writing text-only books (no illustrations) then it's all you need for writing and formatting. Amazon and others will happily process a .docx file and you don't need epub, although there is a free plugin for Word that will create an epub — but the results are generally too flawed for Amazon to accept.

As others have said, cover design is your greatest challenge creatively  because covers are so important to marketing. Gimp is a great tool but with a learning curve. However, you could import a photo of your little finger and Gimp can turn into something bizarre and interesting.

Marketing without cost, on the other hand, is a real obstacle. You need to get engaged in all kinds of online platforms including Substack NOW, because most don't accept newcomers suddenly appearing to promote their book. They expect long-term engagement and familiarity before they tolerate self-promotion.

The last option is approaching publishers who will deal with all the above if your MS is a piece of genius.

I have met people whose only out-of-pocket expense was the art for the cover, and that still holds true.

If you don't have cover design skills or don't want to learn them, consider licensing the appropriate fonts and photo(s) and then turning to a cheap service like Fiverr to get the cover assembled by someone who actually knows what kerning is. A title whose type has been kerned looks way better than one amateurishly self-done. But do not trust a designer on Fiverr to license anything; bring your own licensed pieces to the project.

You also could search pre-made cover designs to find something that speaks to your genre. That would probably cost you twice as much, but usually you can trust that the cover elements have been properly licensed.

Edited to add: If too broke to buy a cover, then read up online about how to take a photo right and make it look good as a cover. Then read up on how to do type correctly, search for free fonts, and assemble it all yourself. Your total cost then would only be time. 

When I used to do AMS, I recall the data lagging. I'd usually wait about a week to see if an ad is selling--and sometimes, unfortunately, even the sale data turns out wrong.

I've failed AMS multiple times in the past and have completely given up with it. I've resorted to BookBub ads as my main source in sales.