One thing to note: it's always harder to maintain your normal efficiency when you brew a bigger beer. There are plenty of tips to get efficiency up, but one thing you can do is harvest more wort and then use a longer boil.
Mashing low will up your ABV, all else being equal, but the way it does that is by lowering your FG and (a little bit) your sweetness as a result. Similarly, simple sugar is almost 100% fermented, so it ups your ABV but leaves FG alone. Thus it produces a very dry beer.
But to answer your question, mashing higher should only barely affect the OG you get on your wort. If you want to increase sweetness, mash higher and use less sugar. You can also use more crystal malts, especially (for this type of beer) Special B.
If you are mashing a lot of grain and are worried about efficiency, you can employ a longer mash. I would also, as I recommended above, use more water and thus get more wort, but boil it longer to concentrate it. And of course you can "cheat" by using a little extract, which will not really affect the flavor of a big beer anyway.
As for what you like, I hear you--there's a whole range of Belgian types. I meant my sweetness comment only in very general terms. I do think that some of what gets perceived as "sweet" in big Belgian beers is just fruitiness. That comes from both the dark sugar (which gives dark fruit kinds of flavors) and the yeast (which produces all kinds of esters). They are dessert sort of notes, but not necessarily sweet. All of this said, Gulden Draak is quite sweet compared to some of the Trappist beers, and of course you should brew what you like.
So with all of this in mind, I'm looking back at your recipe. If you want more sweetness, you can use a pound of Special B. You can also cut all of the brown sugar (probably a good idea anyway). And for the Belgian candy sugar, use 1 or 2 lbs of D-180, the dark Belgian candy syrup. That will give you some of the dark fruit notes you're probably after. Note that dark grain is not the traditional way to get dark colors and flavors--quads are not stouts, after all (nor are they doppelbocks).
Oh yeah. You should also buy a new hydrometer!