Sausage mince is generally beef off cuts with water, spice mix and fat added to it. Lamb and pork off cuts may be in there too.
Rump can be used to make beef mince in the tmx and that mince can be used for meatballs, meatloaf, lasagne, etc as per normal mince. You could use some for sausage rolls but I would add some sausage mince or other fatty mince to it.
I freeze my mince in 500g lots which I flatten out which helps thaw out easier.