Hey there - stop everything you are trying right now and get the CB into its own small tank, then order live black worms here:
https://easternaquatics.com/ (A 1/4 pound will last a month if you change the water daily. Don't use RODI as it will kill the worms - used filtered tap (think Brita) or dechlorinated tap by leaving it out for 24 hours+) Your copperband will likely never change feeding behavior, or be stress-free enough to put on the needed caloric buffer to learn how to eat while in a new display among other fish the same size. It will likely slowly starve to death like many do. Not a certainty, but likely. Live black worms are a sure thing - it will put on weight and live. I tried everything else under the sun - clams, mussels, mastick, other frozen, tubifex, blood worms, the list goes on - it ate nothing but the live black worms. Once it is eating them, slowly introduce a small amount of mysis (Hikari brand are smaller than PE Mysis, and a better starting point). The CB will ignore them for a while, then eventually try one. Once you observe it eating the mysis, you can slowly flip the percentage and wean it off worms. This transition took more than a month for mine. I've had my CB for close to 5 years and it eats all the meaty foods I put in the tank (now a DIY seafood mix of mussels, tuna, nori for tangs, etc.) Some may get lucky, but mine has never touched pellet or flake. For ease of feeding, you really want to train it to eat what you are feeding everything else if possible. I can't imagine having to feed a mussel on the half shell everyday - but my respect to those that do. You can fart around and hope and pray and toy with the fishes life, or you can just separate it and order live black worms and have it live. Sorry to be so delineating, but it is my favorite fish, and too many are lost because people simply hope for the best...