I can't give you links because I'm working on a tablet computer.
Go to Rome2Rio.com, enter Collioure as the origin and Ceret as the destination. Then drill down to get to the links to the bus schedules. Go to the Deutche Bahn website or the SNCF website to check the train schedule. You will note that, as is all too common, Rome2Rio's statement that the train between Collioure and Perpignan runs hourly is not accurate. However, it is reasonably frequent. Obviously, you'll need to check the return schedule as well.
My guess is that train to Perpignan followed by bus to Ceret will work better than the two-bus option, changing in Argeles-sur-Mer. However, you should verify that the train and bus stations in Perpignan are close together.
Whether a day-trip is worthwhile is a personal thing. I don't mind traveling for two hours to get to a pretty town if I feel like I'll have time to see what I want to see. But of course I'd choose an equivalent but closer destination if one was available.
You're running into what is a typical situation when a traveler stays in a charming small town without a car and wants to take day-trips. My general philosophy is that one awkward day-trip is OK, but if I plan more jaunts than that in the area, I prefer to stay in a larger city that has good transportation. I opted for Perpignan.