for the coordinate system got to workflow tab. then settings.
personally i use project and output at the same local UTM system. and then i on export I manually set and x,y offset to re-zero the model.
this way I get the same export position each time.
as for your workflow.
I would add laserscans first, then drone, then ground level.
also after you register scans you should if possible export to many e57 or ptx. so you get one file per location.
if you export just one file, you lose a lot of info and have had problems with that.