Unusual but big problem

Have an 07 c230 sport, bought it roughly 2 years ago. For the third time the flywheel is cracked. Quick back story, I bought it from a Mercedes mechanic who owned the car as his daily driver. He blew the engine, and replaced it with a 3.5L, 2.5L was the size of the factory engine. About a month after I bought the car it devoloped a tap/knock that turned out to be a cracked flywheel, previous owner (a friend) changed it for the part cost. About 14 months later, same problem, paid 900 for a Mercedes specialist to change it again, and not even 4 months later the new flywheel is damaged. I don’t even know what to do. I really love the car, expect for the obvious issue it’s a great car. I don’t want to trash it, but doing another same repair seems like a waste. I always set the tranny in comfort mode, never drove it hard, but that didn’t seem to matter. I’m grateful for any suggestions or advice. Thank you