Get yourself a tourist visa and the 3proofzz. When applying for the visa have your gf down as guarantor. Have her phone number to hand as you re enter.
word of advice, if I may be so bold. Don't go telling everyone you're an ex squadie, they'll have one eye on your pension the whole time. Visa agents are not regulated, not trained and usually become an agent because they have an aunty or uncle working in an immigration office somewhere up north. Don't trust them
Dealing with visas isn't that complicated and if you can manage to use a photocopier and a stapler without injuring yourself then you can do all this on your own.
You need to give more detail. What you're asking about is a mess.
What passport do you have? Where are you? Are you currently in Thailand or are you applying for a tourist visa outside of Thailand? If so where.
You say you've taken a long term lease? What are your plans moving forward?
At a guess I think you're looking to apply fora 60 day tourist visa that can be extended once for 30 days.
You will need an onward ticket out of Thailand within the initial 60 day period. One of the "onward ticket" sites might fit the bill. Don't use an agent. They always lie. Do it yourself and learn by doing.
I don't think you can spend time there. I think it's bounce only. That said I know of 1 Liveaboard that has been to the Mergui Archipelago recently, but it was a group from Asia Geographic ( scientists)
There was a recent update on the Ko Phayam page. Andaman Open BUT no immigration there. Ranong Kawthong open... it's as you described but you need to provide a 100thb bribe on the Myanmas side