With the low cost & power of modern microcontrollers, instead of having the dongle act purely for licensing purposes you could offload some of your "secret sauce" to it (I assume your software does a lot of calculations with some hardcoded, industry-specific constants). This makes it somewhat crack-proof because cracking it would involve replicating your secret sauce - at which point they may as well just make and sell their own software instead of distributing cracks.