It seems like you are on the right track. I've had the best success feeding them their favorite food several times a day to fatten them up and get aggressive appetites - to the point they "pounce" on the food. Then I switch food soaked in the juices of their favorite food and drop in only a couple of pieces. They instinctively pounce on it. They might spit it out swim around and come back over it and try it again. I repeat this until they readily accept the new food. It seems to work better with only a few pieces at a time instead of dumping in a lot of the new food at once.
Once they have accepted the new food, don't offer the old food for a while or they may switch back to it and you have to start over again.
I've had the least success with the starvation method as the ray seems to lose its appetite. I don't know if it's because their stomach shrinks and they get weaker or what. Unlike other animals when they are starved that will eat the first thing offered to them, rays seem to do the opposite. If they don't recognize or associate something as food, they often will skip over it.
Often you have to train them to eat the foods you want to feed them.
