Whats a hobby/craft that you wouldn't expect that has an incredibly high ceiling either monetarily or in sheer skill?
For example, I'm sure the average joe doesn't know just how expensive calligraphy pens can be, or how deep the rabbit hole goes on video game speedruns.
We had one for years. I cleaned it one day just like I had done a hundred times before. The next day the water turned cloudy and all our fish died. Sold the tank and cabinet a few days later. Having an aquarium is a 2nd job.
We had all the fish die in our aquarium once when I was a kid. Was random and unexplainable at first. Then I found out someone had run a powerful ozone machine to combat mold in the house and no one thought about the fish tank.