1.8 kg = 4 lbs LME. That will make a pretty weak beer. Add 2 lbs of cane sugar (and it is OK to use cane sugar) will get an OG around 1.047. If you end up at 1.012, it will give you about 4.5% abv.
Personally I like some sugar in my beers to help dry them out, but at 40% of the fermentables, it seems way too much.
I think I would either replace a pound of the sugar with a pound of dry extract, or just use 1 lb of sugar and reduce the volume to 4 gallons.