I agree, if the “reward” is causing a meltdown, it’s not the right reward. If he likes change, I think that would be a terrific replacement for tickets and something to certainly bring up with the therapist.
One little child in my son’s group L-O-V-E-D to collect legos. Instead of something like tickets or stars, he got legos. Like seriously, a lego at a time. The child absolutely LOVED it and completely responded to it. My son responded to just getting smiley faces and a certain number of smiley faces got him a reward that he got to pick out. Our therapist was very willing to tailor the program to meet the individual needs of each child.
You’re not a terrible Mom. Every kid just seems to respond a little differently. I’d see about the coins. I’d start with small value coins first though so you don’t go bankrupt. LOL.