Are you aware that the STV cannot be extended in any way beyond the scope of the STV scheme. And arriving July 3rd will mean that you can't extend even within the STV concept. Maybe go for the 60 day TV and possibility to extend one time. Atleast with that there may be options to extend in country.
You can get the covid extension or visit that family extension... both are 1900thb. covid extension is easier. You'll need a contract for your accommodation, don't remember anything about 6 months but if that's what they ask for then that's what you'll need to supply. I'm sure you can fudge something with the landlord. Atteched is the requirements from 8 months ago from Koh Samui.. You basically need 2 * files. best of luck.
Yes. Tr visa gives you 60 +30(one extension). Visa exempt currently gives you 45 days + 30 extension (normally 30 + 30 but extended temporarily to cover quarantine).