Paint mesh like terrain in Unity

Hey guys,
I want to build a huge island in my game. So far I have a really rough mesh in Maya, but the problem is, my island has caves (which Unity terrain can't handle)

Is there a way to paint meshes like a terrain in Unity? I've seen Polybrush, but it's still in Beta, and I doubt it can handle a mesh the size of what I'm looking for. Surely I can't be the only person who has had this problem.
Island: https://ibb.co/efCbDz