I was quoted 35,000 Baht. This included opening a bank account.
I did it all myself for a fraction of the cost.
Opening the bank account is the tricky part, other than that it is just a case of following a simple process, filling in a few simple forms and providing the evidence immigration require.
It's your choice.
There are lots of helpful people in this group that will help with any questions.
I wouldn't recommend they live on any island as I think it would get boring, but they seem to want to. I think you are correct about Koh Chang being too small.
I am not aware of any advantages other than it being easier John, but I read a lot of posts from people saying they were giving up on the marriage visa and reverting to retirement as it's too much hassle.
I have no personal experience, so it was just a suggestion.