Is my Thailand visa still valid if I leave at 13:30 on December 30th, or will I incur an overstay fine?

Keep in mind a 30 day visa exempt entry stamp actually stamps you in for 29 days because they count the day you arrive as day ONE (no matter how late you get in that day).

Also keep in mind that IF you are flying out, and your overstay is under 24 hours the airport will waive the 500baht overstay fine and they won't put the half page overstay stamp in your passport either.

All you get is a thai worded stamp next to your triangle departure stamp that says "overstay of under 24 hours fine waived"

Like this

Using the family brain cell again? Thai visas are valid over a 24-hour cycle as in any normal system, so yours is still valid till 23:59 of December 30th. If you are leaving at 13:30, you still have 10.5 hours to spare.

Check the date that was stamped in your passport, you need to pass immigration at worse at 23:59 that day.