I'm an idiot at times.

Something I never tried was changing from Sculpt to Model mode, which means everything works how it should, and I can mirror the body and adjust proportions, with no need to convert to T-splines, which Sculpt mode relies on. I wrongly assumed that because Fusion opened the various file types in Sculpt mode, I had to use it.

As a further discovery, Mesh mode handles the STL fine, however it's got no option to mirror anything.

This is part of me seeing how capable Fusion 360 really is, but I'm still not sure I'd trust Autodesk with keeping all my files secure in their cloud. BobCad is the other option I'm considering, but it gets a bit pricey to get adaptive toolpaths, but at least it's not subscription based.