To answer the question you actually asked, Sinovac is the only vaccine available right now. Astra Zenaca will be here in a month or 2. Vaccines being offered to Thai nationals in hospitality and tourism first and foreigners holding work permits and under 60 years of age. You will not be able to get Astra Zenaca right away.
If you pay the overstay and it is the first time, no big deal. But allow a bit of extra time for processing at the airport. Just explain why. They will understand. Keep the receipt.