I'd definitely recommend some kind of sparge. I do no sparge BIAB, but I use a 10g pot, always start with 7.5g of strike water, and I can get about 18lbs of grain in it without getting perilously close to the top of my kettle. I don't know if you can figure out a ratio from that info, but I don't see any reason why you wouldn't be able to BIAB that amount with what you have, you'll just have to sparge it. I don't have any experience with that process so I can't help with pointers there, but when I imagine the process, if you have a 2nd kettle that is big enough, dunk-sparging sounds easier and faster. If you can have your sparge water heated up and ready when your mash is over, it would be a piece of cake to pull the bag from the mash and drop it into the sparge water. Then you can turn the main-kettly burner on high to start your boil while you dunk/stir your sparge pot. When ready, pull the bag out and then dump all water into the main kettle.
Pouring hot water through a heavy grain bag sounds like a recipe for a big mess based on my experience, but if you have the right setup for it, I'm sure it works fine too.