Lyman’s law is the only rule though, and I’m not sure how helpful that is cause it can only tell you for sure when a word can’t have Rendaku in one situation.
I feel it’s true we all gain experience of whether a vocab is likely to rendaku or not, tgerefore there must be a reasonable set of rules that give a decent guess. It would be a great start to code up the rules that are around, starting with lyman’s law, and so find the exceptions.