If you transfer to Chengdu, it will take about ten hours, and you don't have to wait for a bus in the middle. Even if you go to Dazhou by train after Chengdu, the train to Dazhou needs to go to the East Station, and you have to wait for a bus for an hour in the middle, so the time is similar.

Another way is to take a bus from Guangyuan to Ankang, and then from Ankang to Dazhou. This, plus the waiting time, usually takes only eleven hours. In Ankang, it takes only one hour at most, and it is a transfer at the same station. It takes seven hours from Guangyuan to Ankang, and three or four hours from Ankang to Dazhou. There are many shuttle buses from Ankang to Dazhou.