Referencing an official government announcement, the Travel Authority of Thailand states to enter Thailand you need "A Certificate of COVID-19 Vaccination/Recovery". And "Everyone 18 years of age and older must be fully vaccinated for COVID-19 with an approved vaccine at least 14 days before travelling to Thailand."
Is Astra/Zeneca on the approved list?
Does one need a booster to be fully vaccinated?
https://www.tatnews.org/2021/12/thailand-reopening-exemption-from-quarantine-test-go/