There is no minimum time to return to return to Thailand.
If you fly out, you can take the first possible flight back into Thailand,
If you walk out by land, you can directly turn after being stamped in/out by neighbor country, but be aware you if you do this it is only twice a calendar year using land borders.
I'm looking around in the reasons to apply for a Non Immigrant O visa as being over 50 years of age, but this is the only option OR change into the long stay visa.
Alternative, the Single Entry Tourist Visa, get 60 days in Thailand, arrange a bank account, transfer the 800.000 Thb and apply for conversion at the local Immigration Office.
Yes, you can convert the Tourist Visa into a Non Immigrant ED visa in country. But this information can not be used to ignore requirements for the Tourist Visa.
a 7 day extension of stay does not exist in Thailand.
If you apply for a new Extension of Stay and pay the 1.900 Thb but your application is not approved you will get a new stamp with 7 days to leave the country.