Both Sinovac and AstraZeneca were being used from end of February 2021, though the amount of AZ was only what could be supplied from outside Thailand. Locally produced AZ became available from 6th June 2021. The mixing of the two vaccines started 14th July 2021.
However, vaccination rates didn't pick up until August September 2021, there were still only 1 million fully vaccinated people as of end May 2021. But by September over 2 million were being added per week, by December over 40 million people had been fully vaccinated. The current figure is approaching 49 million.
Sinopharm was available from end June 2021, Pfizer from July 2021 and Moderna from November 2021.
https://en.wikipedia.org/wiki/COVID-19_vaccination_in_Thailand
https://dashboard-vaccine.moph.go.th/dashboard.html