Hey Aininia,
Perhaps some of the faces are not connected to one another and there is a lot of degenerate polygons?
If you have Blender, open the meshes, go in the modifiers and apply a “Weld”. Up the Distance in it until its high but doesn’t change the geometry visuals.
That should remove any overlapping vertices (resulting in degenerate faces).
Apply modifier and then export.
If that doesn’t fix it, then perhaps its indeed because of the low amount of polys. Try to apply a “Subdivision” modifier in Blender and give it “Simple” setting instead of “Catmull-Clark” (secondary would make them "smooth in geo, while first will keep them as they are - low poly looking).
You could ramp that modifier up to 1-2 levels, get yourself some more geometry and subdivisions on the models through it and then apply it and export and retry Nanite.
If models are heavily triangulated you would want to see if you could easily run a “Tris to Quads” before adding any modifiers.
I mean I know this is not idea as at this point you might just as well recreate the meshes, but I hope it helps in some way as an idea.