I think you're right to discount a Latin origin, and to consider the possibility that the name has been shortened. There is a name Doxopoulos by the way. Doxa means glory or fame, as you point out, but I imagine that the word is feminine in gender and would give a female name. Are there any Greek surnames based on female forenames? I can't think of any offhand. Maybe the S ending makes it masculine, I don't know. I've never heard of anyone with the forename Doxas, or Doxa for that matter.