I can't tell you if immigration will ask to see your exit flight ... but they MIGHT. If you're worried, maybe buy a dirt cheap flight to a neighbouring country and either lose it, or get a refund/voucher after you're in ASQ.
My Thai partner flew back to UK last weekend. Aside from and required visa/BRP, they needed a negative covid test of any type within 72 hours of departure and they needed to complete the online return to UK form and be able to show a QR code that this generates (can only be done within 48 hours of arrival). Both can be printed or electronic.
This is asked almost daily. Go with a Thai person and try different banks or branches of the same bank. I've even opened one whilst on 30 day visa exemption.