Hello wojtek,BigTIFF export option is already available for Orthomosaic and DEM export.
Have you tried export TIFF with deflate ? If you export as a TIFF with None compression. it is hugh file size.
Hello wojtek,Have you used BigTIFF textures with common image and model editing applications? We have not performed such tests, but are not sure, that such data type is properly supported by many applications of these types.