You need to show for the Extension of Stay a monthly income of minimum 40.000 thb as a foreign deposit into your Thai bank account for at least 12 months prior application date.
Yes, you can apply in Phuket during the last 30 (edited) days.
No health insurance needed for the Extension of Stay, if the insurance covers the requirements of the Thailand pass is something you have to ask your insurance company.
As soon as the new Extension of Stay is stamped in your passport, you need a new Re-Entry Permit to keep this extension alive when travelling.
Time to stop this, sorry but I never seen assistants helping to carry the tons of documents transported in 18 wheel trucks to apply for an Extension of Stay or a visa.
I understand 1 mobile phone with an app for pictures should do, but this is Thailand and Immigration want to see some hard copies.
I will open this for the person who sent me the list with at least 100 different requirements needed to apply for any visa or Extension of stay.