As far as I know the further south you are the later the monsoon ends. Therefore it is probably not a good idea to go to Panama or Costa Rica in September and October. Instead we probably have to concentrate on Mexico/Belize/Guatemala.
Do you know how the weather is going to be? Just rain for an hour or two or is it pooring down the whole day? If it is too bad we'll go to Baja California instead. No rain. But therefore "just" desert.... As we would like to cycle and camp along the way it might be the best option.
What do you think?