FBX export appears to have a file size limit in that the process will fail at the last leg having produced part files in the given directory.
I have a drone + land model which was 175M tris, this failed to export to FBX but was successful using OBJ but it was a 30GB file. I did load this into Autodesk Recap Photo in the hope that I could then export as an FBX. It took an awfully long time to convert to RCM but the export to FBX was taking forever so I abandoned the process.
I did mesh the dense point cloud in Autodesk Recap but the result was poor and minus the textures.
SIMPLIFY is the answer, 100M tris works. The resulting FBX is 3GB which is way under the upload limit of Autodesk Drive which is 6GB. I recommend Autodesk Drive for a simple on line share of the model.
So somewhere between 100M & 175M tris is the FBX limit and one which results in a file under 6GB. It would be useful to find the sweet spot so we can upload a model at best possible detail. Though 3GB is taking a while to load into the online viewer and once it has there is a serious lag.
PS Don’t texture until after SIMPLIFY and any other model adjustments or you will be doing it twice.