If fan fiction makes you happy, I don't see the need to justify it since it harms nobody and satisfies you and some other folks. I think it makes sense that you write about a shallow story and characters so you can flesh them out. I know what you mean about feeling paralysed about creating characters from scratch. I'd have the same feeling every time I'd have to write a paper. I'd look at the blank piece of paper and think "How can I ever write anything that the professor will think is interesting?" That feeling of paralysis but wanting to be creative led me to MUSHing, which is rather similar to fan fiction in a lot of ways. MUSHing satisfied me for a long time, and I might have continued to write as a hobby for years if the gods had woven my life differently.

I started writing fiction when I realised I could draw from the themes and characters of my own life without actually writing about my own life. In talking to you about your own life, I know you have a lot of interesting stuff. I'm sure you're drawing on it already when you write your fan fiction. If you haven't already read it, I highly recommend Bird by Bird by Anne Lamott.