I've used one of my fantasies. It's, IMO, my best novel and took me four years to finish. Ironically, it's doing the worst in the market at the moment. But I like it because it's a fantasy and my upcoming series is fantasy (I also write Sci Fi, btw).
I don't write prequels or short stories just to use for a reader magnet. I tried this once, but, for me, found that my writing was contrived and awful. I have to just write and use what I have. I can't write just to market. But that's just me. So, I use works I've already written.
There are other issues. Not all my writing is the same. The character in my magnet novel is very conceited and sure of herself, ironically the opposite of my current book series. So, I don't think everything fits. But I do think that the fantasy matches my current fantasy genre and hopefully brings in readers with fantasy as an interest.
I use my reader magnet through Bookfunnel and Story Origin to get more subscribers. Hope sharing my ways helps.