Curiously, since it’s not relevant, Wookiepedia never mentions a Planet of Origin for Caleb Dume
To supplement speculation that Caleb Dume didn’t find his Kyber crystal on Illum but on Jedha, the Jerusalem of the Force.
What if Caleb Dume’s birth planet was Jedha? What if he was born to Guardians, who deemed it a true honor that their son would not be a Guardian but a Jedi?
Or what if Caleb Dume was an infant Jedha orphan discovered by Jedi?
What if in these scenarios, he had minor encounters with a Chirrut, a young Guardian-in-training who looked after the orphans and interacted frequently with the Guardians.