Totally agree with @Enchanted_Elixir. Bear in mind that it's Thailand so better to relax, go with the flow & not try to control everything. If you find a masseur who you like & have a rapport with, he may be better able to address your particular requirements. Very good point about the Saphan Kwai shops (like 15M) & how much massage is going to happen if you are interested in penetration. Prince Spa might be a good place to start.
Anyway, good luck & I hope you enjoy your first visit to Thailand.