one of the main reasons I want to go to Thailand right now is because they have control of the situation. If they open without quarantine I wouldn't bother going, seen as they'd end up in the mess we find ourselves in Europe/US with lockdowns and restrictions.
i had 2 flights cancelled in April (Emirates/Aeroflot) and got a full refund, maybe try to do a chargeback on the card you paid the flight with if they are refusing a refund. There are also services like
*************
that help get refunds for a fee, i haven't used that but there are many like it that might be of help.