How to properly mirror an object?

When I do it through object >mirror it comes with inverted normals and wrong tangent basis when exported from Blender. When I do it through mirror modifer it’s fine but I have to put one more boolean to delete a half . Is there an add-on or something that just makes a properly mirrored copy ?

Of course if you mirror an already completed model you get “doubles” and the mirror modifier is for mirroring one half… you may want to have a look into Edit mode (select all wanted verts,edges,face) → Mesh-> Symetrize ?

I need a way to mirror a whole collection, not just single mesh. So wonder is there a simple way?

Maybe some script that fixes geo after object> mirror?
Surprisingly the mirroring always looks fine in Blender view ports . Not after fbx export whatever settings combination I try .