I heard that nowadays it is very easy to extend your stay at any immigration. But better to ask, what documents are needed (from immigration to immigration different) to avoid to have to go twice...

many airlines offer flexible tickets during the pandemic; return leg can be postponed...
another way would be to buy a ticket to one of a country in the region and get a refund or if a budget carrier, dump it...

of course you also need a COE to enter Thailand