How can I make this sculpt manifold?

Currently trying to make a 3d sculpt of my friend’s dnd character to 3d print, I made it this far but am having a lot of trouble making the sculpt manifold so that I can print it properly.

There are 1030 Non-Manifold edges, and after the different approaches i’ve taken after searching online, the result is the same every time – the back of the head gets completely wiped. Is there anything I can do to fix this? Did I just sculpt this wrong? I used the boolean union modifier to join all of my different objects, (shirt, hair shapes, facial features, etc.) is this the problem? Should I go back to a previous save and connect the shapes differently?

Any help is appreciated, thanks.

Ouch, afraid you’re going to have to retopologize the mesh.

Anything’s better than scrapping it entirely. I didn’t even know retopologizing was a real word, so thank you. I’ll try it right now

Try Instant Meshes.