Oracle Database Upgrade Assistant "Failed in bringing up..." Error
So I found the solution after much investigation and trial and error. I deleted the .log files from $ORACLE_BASE/cfgtoollogs/dbua/logs. After restarting the session and the UA, it worked fine. I'm not entirely sure WHY that worked (which bugs me), but there's the answer. Hopefully this will be helpful to others in this situation.

Thanks to those who commented on my question.