Even though freedom units don't come naturally to me, this sounds like extremely poor efficiency for what you're trying to achieve. Or are you aiming for a pastry stout with an OG of 1.180-1.200? In that case think thrice about using an AIO for this recipe.
Regardless I'd go for a double or reiterated mash, maybe even triple. In a nutshell: Mash 40-50% of your malt, sparge until you get back to the correct amount of brewing liquor and mash with the rest of your grain. Then mash, sparge and boil as usual. This will get you a very decent efficiency for the gravity you're trying to achieve. You can vary a bit with what you do like adding 30-40% of your malt early on to maximise your sparge efficiency and doing 60-70% in the second. You can also add roasted malts later for better efficiency and managing water chemistry. I've gotten 70-80% efficiency on high ABV beers using an AIO this way. You will probably have to boil quite a long while and I tried many things to maximise efficiency. You might want to consider adding malt extract, sugars and or maltodextrin to get the OG/FG that you're aiming for. If you can post your recipe and are interested in extra pointers just let me know. I'm interested in a recipe with this grain bill any day.
Edit: this all advice on the hot side, but there are also plenty of things to manage on the cold side. Like (step) feeding sugars, pitching enough yeast, keeping yeast healthy and having immaculate temperature control.