I have to admit that travelling in Chanthaburi without private transportation is not so convenient. This might be one of the reasons why Chanthaburi is not so popular among foreigners.
There are schedules for the bus but you cannot count on them. The bus might be delayed or some round could be cancelled, if there are not enough passengers. For the public van, you may have to wait until it is full before it gets off.
So mostly, Chanthaburi is a heaven for travelers who have visited Thailand before and travelers who enjoy a slow life. Take your time. It’s a holiday!!!