Coming out of ASQ you can go anywhere. Generally there are no domestic travel restrictions. There was an outbreak of Covid yesterday in Bangkok though and 4 cases traveled to CM. It's fluid, things could change, but coming out of ASQ you're fine.
Long story short, you need proof of a ticket leaving Thailand when traveling there on anything other than a long term visa. Most countries have this requirement, it's usually enforced by the airlines as they are responsible to return you at their cost if the country refuses you entry.
Don't believe they are extendable. APEC entries are equivalent to visa exempt entries for applicable countries, and visa exempt entries for 90 days are not extendable.