Can a DM class be fun? Sure. Will you learn some new skills? Probably not directly. You'll work on core recreational skills so you can do those to demonstration quality... You can spend your time in the water working on things like trim and positioning, but you'll learn far more from a properly taught cavern course.
I'd be suspicious of anyone who would let you take a cavern course in a single tank though. A single tank is within standards for most agencies, but if an instructor is willing to teach in a single tank, that probably isn't the best instructor for a cavern course.
OP is a recreational diver. If he doesn’t want to go tech, yet improve his skills, a single tank cavern course is a good option. Forcing someone to go doubles or sidemount just to get advanced training is stupid, when they wouldn’t be diving that configuration at all in the future.
Bill Ostreich (sp?), a long time FL cave instructor, teaches cavern single tank. I know people who’ve taken it with him. My Mexican full cave instructor teaches cavern single tank to recreational divers with no wish to go tech.
People can take Fundies single tank, don’t forget that.