if you get Non-O (90 days) in your home country it is pretty easy to open an account upon arrival to season the funds for 60 days to extend the visa for a year at the end of 90 days.
First year extension: 800k deposit only option. 2nd year extension 65,000 monthly foreign bank deposits onto the Thai bank ac for previous 12 months or 800k seasoned for 2 months. Do not forget to fulfil the term of the 1st year extension to maintain 800k for 3 months and min 400,000 thereafter. Switching to 65,000 monthly transfers can be expensive during year 1 as must maintain the balance (for year 1 extension) as well as make regular monthly transfers of 65,000 (for year 2 extension).
3rd only remitted money to Thailand is taxable in Thailand , not earned globally .. and 4th to take the double taxation treaty into account (if in place)
Much easier to do it in home country. I tried to do it in Thailand but was so cumbersome (without an agent) that i have done it in home country instead and not used agent for in country extensions either.