Pointers for Kindle Fiction publishing?

I've written 3 cozy mystery Kindle ebooks that are 90K words each. Combined, they do about $18 in sales a month, which is pathetic, but I haven't done any marketing (They started earning $60 a month but then came down to $18). I do, however, get a couple of emails every year from readers who are eager to read the next instalment.

The trouble is that I am a slow writer. It takes me about 10 months to write a book (Cozy mysteries require a lot of work to set up and resolve red herrings).

I changed tracks and decided to do a romance book, as they don't require as much planning. This time I developed the concept but hired a ghostwriter to produce the book as I figured it would be quicker. Finding a writer that stuck to my concept was hard work (nearly as time-consuming as writing a book), however, I eventually found one, who delivered the finished manuscript in a week. The work produced was weak, but re-writing it into something decent only took a fortnight. Clearly, I am much faster re-writer than a writer.

This single book does about $10 month in sales (after the initial launch where it did $150), which is also pathetic, but again I have done no marketing. As before, I get emails from readers who are keen to read more.

I got sidetracked by life and so haven't done anything book-related for almost a year, but I would like to give it another stab to turn it into a viable business.

The areas I need to crack are:
1) Hiring ghostwriters
(so I can turn around books more quickly). They don't have to be great writers, as I will be re-writing their work, but they do need to stick to my concepts.
2) Marketing my books.
There's a lot of information on the internet, which is often contradictory. I've tried some of the free marketing options such as submitting to book sites and notice I get a tripling in sales when I do this. However, I am not sure what paid solutions to use.

I would appreciate it if someone could give me some pointers. Thanks!