i cook my lamb for 3 hours for that weight.
i start at 200c for 35 min to seal the outside.

then i cover in foil with a little bit open to allow some steam out for 2.5 hours at 170c usually cover the little opening for last 40min or so.
non fan forced.
remove 10c for a fan

then i let the meat sit for 10 min fully covered in foil

and i leave my veg in for the same time as well
im an all in oven at one time cook lol