You can sleep on the beach for free. You can pay someone $1000 to cook you a $100 steak while you sleep on the beach for free. A very common expression in Thailand it's all "Up to you".
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<- that's a pretty good site to find local prices. You can get cheaper if you want.
500/month walking around money, its ok. You can have a good time that you will remember the rest of your life. If you're some working stiff and this is your big once in a lifetime get-away.... I would say, go for it.
However, I think you maybe over budgeting on your 'mandatory'. I don't think you have the money for "island hoping" but you probably have the money for 'Island staying'.
Koh Samui is a large lsland, you can visit a couple of the smaller islands for the weekend. You could get monthly rental off the beach for much cheaper than $700/month. A motorcycle rental is $150/month.
You can do similar for Krabi. If you rent on the beach $$$$ if you rent 2 or 3km from the beach $$. If you have a Thai companion $ (because they know their country better than we do).
I use google voice. Works for most things but I have some issues with the more serious verification stuff. You would need to use your friend's phone number and they'll have to verify every 3-6 months.
I have pulled visas from the Thai Embassy Washing DC while in Thailand. I then exit and return. You can pull a visa locally or from the US. For me it was 1 week. But that is no guarantee of entry, they have been cracking down.
I would travel to a land crossing and cross by land. I think that would be less of a pain in the neck if you get denied.
I think I just paid about 30,000, maybe a little less. I did not have the 800K so the agent provided that. I had to show up at immigration one time to have picture taken. I had to show up at the agent's office 3 times, I think.
"You are not covered if you are outside the US for longer than 6 months in that year"
How do you get around that part? My US job provides insurance which I think is 100% useless to me. Currently, if I take ill and need serious medical help, I will have to travel back to the US and pretend I got ill there.