An actually good backstory, AND THIS WAS FROM A ONESHOT!??? Author, you have my applause.
The oneshot had basically no deeper backstory or characterisation, and was short enough to not get overly caught up in itself, so going from there to here was relatively easy - the setting and character development is almost entirely built for this serialisation.
None of which is a criticism - a oneshot
should be tightly focused, only presenting the bits that are important. A oneshot is a short story, and the key thing about short stories is that they're
short - if there's too much in the final version of the story then it ends up muddy and confused, the true art (and greatest challenge) of the short story is cutting out everything that doesn't explicitly need to be there, without cutting any of the bits that
do need to be there. Which really is amazingly difficult to do well . . . but it also leaves things very open when it comes to a longer work inspired by a short story done well - there's so much less detail tying you down and constraining what you can do.