Yes the Thimble Cactus is incredibly slow growing, the arms only grow a tad longer this past summer but it did thicken at the base a lot. It was pretty small when I purchased it 2 years ago, it also had rust or some sort of plant fungus upon arrival. Thankfully I got that cleared up and it's super healthy now. I think come this year it will start growing faster now that it is healthy.
Speaking of purchasing plants and having them die, I am so sorry Fluffy.

It's the risk we take when ordering plants sight unseen. I lost quite a few that first year, that came with rot, fungus, over watered, bugs, etc... It's heartbreaking to lose them I know.

I doubt over potting your plant would have killed it this fast, from what I can tell with my limited experience, succulents can take an awful lot of abuse before dying, UNLESS they are already on edge. Your plant was probably already struggling and probably would have died anyway no matter how you potted it.