A key-generator outputs an encrypted string that only changes the second half - what weaknesses could that expose?
The reason what you called "real licenses" look like ssh-rsa keys is probably just the encoding format for some of them. ssh-rsa keys are base64 encoded, and that might be the only reason. The decode content can really differ based on the license checking algorithm.

You'd probably be better off disclosing your license verification algorithm, and letting people comment on its security/strength which will enable you to improve it in case any issues are found. The fact that the licenses start with the same prefix, on its own, doesn't indicate much about the security of your license verification.
You should also be clear about the assumptions you're making about a potential cracker, and what can they do. That would help assess whether the licensing scheme is secure under those assumptions.