How to get an entity create route, given an entity type and bundle?
Given Clive's comment, and some further thought about it, one thing I could do is create an annotation, such that custom entity developers can declare their route. Further, I can add annotations to the module that cover all core cases (node/user/taxonomy), such that the module works with core out of the box.

From there, I could add conditional logic that looks for a given entity's annotation, and if it doesn't find it, it would then make the same assumption (/{entity_type}/add/{bundle}) as above. I suppose from there either developers can declare an annotation in their own module, or if a particular custom entity is popular and keeps hitting the issue queue, I could add that as well.