I’ve had a mandarin dragonet for over 2 years in my 90g - first and only one I’ve had so I can’t speak to other experiences. Mine doesn’t eat any dry or frozen, only live copepods. Since I run a sumpless system, I created a copepod “condo” in my tank which is basically a plastic breeder box with lid that I filled with LR rubble, sand and a clump of chaeto and it sits in the bottom of my tank at the back behind rocks so it’s not visible from the front. Kind of an “in-tank” refugium of sorts. I “seeded” the tank in advance of getting the mandarin and the condo allows for a safe reproduction of copepods as preditors cant get in and the pods just swim out into the tank to keep the food chain for the mandarin supplied.
I was nervous about the fish surviving as I did try to convert it to mysis but it never ate it. It looks healthy and plump so all must be well 2 years later in the condo. I can monitor the pod population by looking in the condo to see whats scurring around. Its always pretty active in there

Good luck if you choose to get one, they are finicky but once established low maintenance and beautiful to watch.