The bus or minivan to Kanchanaburi is easy and quick. From the new South bus station. The bus from Kanchanaburi to the border is a very old, very slow bus. Maybe there is a minivan or taxi.
I had a visa exempt stamp which had been extended at an immigration office in Thailand, for thirty days. I stamped out of Thailand and into Myanmar. Then back into Thailand on a new thirty day visa exempt stamp. This is not a visa.