In addition to the above, the book should appeal to young people of all races and genders.
I am a lousy reader. I also read ebooks so i have NO idea how long they are. But from my notes i can pull up these:
I have notes about Join, and can't recall it (clearly?): https://elainegrey.dreamwidth.org/680396.html Also don't know about "not depressing."
The Ancillary series is probably longer than desired?
Jemsin's The Broken Earth -- is that more fantasy?
Becky Chambers' Wayfarers series has great world building and is about identity in many ways -- seems good for younger folks.
Any other ideas?
This is also posted at https://elainegrey.dreamwidth.org/773867.html .