Also Unitys shadow mapping is by no means perfect and the Ogre implementation is also not that awsome if you ask me. It is just that shadow mapping for the sun is not easy to archieve at a high quality and good performence. I had something working quite nice already, which thanks to jcl now also works without an engine extension and with view frustum culling. I just got a bit away from working on it due to other projects. But I think that it just needs one problem to be fixed until it is well usable. Unhappily I already messed with that problem for quite some time now and still don´t really know how to solve it.

Why not embrace Ogre, hire experienced users from the Ogre team/community , there are commercial engines already doing that. This could even remove the windows dependency of the 3dgs.