Eh, ask the author to split it in reviewable blocks if you think there's a chance you actually want a version of the code. More likely if it's introducing tons

Eh, ask the author to split it in reviewable blocks if you think there's a chance you actually want a version of the code. More likely if it's introducing tons of complexity to a conceptually simple service you just outright reject it on that basis. Possibly you reject it with "this seems more suitable for a fork than a contribution to the existing project". After all there's probably at least some reason they want all that complexity and you don't.