You should consider writing hard science fiction if…
You love reading them
You have enough scientific knowledge to make it believable. Writing hard science fiction opens you up to attacks of failed realism like few other genres do. Having the chops to back up your writing is of vital importance.
You have a provable connection to the sciences i.e. a degree or occupation in the sciences. As in nonfiction, having provable expertise helps in marketing this genre.
You should probably try a different genre if…
You have no connection or interest in science. You may have an idea for a hard science fiction book or series, but if your science isn’t up to it, choose something else. There are “softer” SF genres you could maybe fit it into.
You prefer character-driven books over concept-driven ones.
The formula for success in self-publishing for a hard science fiction writer is pretty straight forward.
Have a good book. In this genre, that means a book with compelling tech or scientific concepts as well as characters.
Build an ARC or Beta reader team. Get feedback from the beta readers to improve your book and use the ARC readers to get lots of reviews for your book when it launches.
Make sure your title, cover, and description are true to the genre. Hard SF is often defined by its subgenre. Make sure your promotional and marketing materials fit with all your genres.
Advertise to the people who read books just like yours. This means product targeting with Amazon ads and keywords for books with covers that look like yours and similar subject matter. Pick your favorite hard science fiction author and target them directly from the day you start writing all the way through your book launch and advertising campaign.
Keep writing. Don’t rest on your laurels with a good book. Write another one. And another one. And so on. In science fiction, you have the whole universe (or at least our solar system in hard SF; okay maybe just a lunar base?) to explore, so a series should be no problem. The next book helps sales on the earlier ones and it adds up to some serious sales numbers by the time you get to book number six or seven.
Build your mailing list. Readers are always hungry for more books. If they like your stuff, they will snap it up the day it comes out. The same is true with building a social media following.
The hard part of all this is writing a good book. Check out the science fiction tag in articles for more information.