Many people book separate flights on separate tickets. And then you have to go through immigration to pickup your luggage and check in for the next flight.
The OP mentioned in his question that she has to pickup her luggage in Bangkok .....
Try to contact quarantine hotels near the airport and ask whether they offer a rapid PCR test. It will cost extra of course but might be a possibility to get onto that flight.
To be able to pick up the luggage you have to pass immigration. She needs a Thai Pass , Test and Go, and the flight to Cambodia shouldn't be earlier than 24 hours after arriving in Bangkok.
You entered with the 30 days Visa exemption, have done your tourist extension and can now apply for a Covid extension. Important to know that the tourist extension which you already got is never denied , but the Covid extension might not be granted why everyone should have a plan B, just in case ....
At the end of the day either you yourself or someone in your name has to go to the nearest immigration office and ask those questions because every immigration office has unfortunately different rules and asks for different documents.
I am not a Thai Visa expert..... explain to me why a husband of a Thai national who wants to stay longtime in Thailand has to use COVID extensions to do so?
There must be another type of Visa för that purpose?!