Let me just say as interesting as this was, it would have been sooo much more interesting if The Master was the Timeless Child:

• Why is The Master unstable / evil? He subconsciously remembers all the experimentation done on his previous incarnations as a youth and he's now broken mentally.
• Why has Missy's character growth been undone? Finding out that he was betrayed by the Time Lords for a second time drove him to find revenge.
• How does he keep returning from inevitable demise? He has unlimited regenerations.