Is kbin a good place to set up shop? I don’t really understand the advantages/disadvantages of different instances.

Sure! If you like that more. You can still join lemmy comms through it after all (and vice-versa!) Personally, I prefer lemmy because it's closer to what reddit is/was

what are the “reputation points” in my profile?

Only a kbin thing afaik, doesn't affect your profile on lemmy.