I've made my first batch of base in ages using mild Dutch onions (the big ones that normally come in threes in smkts) and there was no sweetness at all....
If you're not getting any sweetness then you're not boiling them long enough.
From experience I've found any type of onion will break down into sugars if you boil then simmer them long enough, when this happens the water turns a white creamy colour.
The thing with a base is that once you add spices initially, then it's hard to see this change due to the colour of the spices mainly tumeric.
If you're not sure just boil the onions until the onions break down as described above then add the rest of your base mix and simmer for another hr or two.
Or just do as I do and simmer the base for at least 4 hrs, then leave to stand over night cooling down slowly before blending it.