Should I hike the Kumano Kodo with a guide or self-guided?

Both work. The Nakahechi's main sections are waymarked every 500 meters and has lots of signs as well. Self-guided walking is straightforward.

A guide adds efficiency and a guarantee of not getting lost on the more remote sections. And of course they offer interpretation, information, and a window into things you otherwise would have missed.

A hybrid approach is usually the best structure. Guided on the culturally dense or navigationally awkward days, independent on the well-marked stretches. For travelers who've walked the Camino de Santiago, this preserves the self-directed rhythm while opening doors a fully independent trip can't reach.