If I had loads like that to carry often, I'd get a trailer or an Xtracycle or go all in and order a Big Dummy. Or a bakfiets. I would also be getting some gears on the bike.

Nice that the bag can hold so much but it's not a good idea to have that much weight on your back when you ride. Center of gravity is too high and unstable. As you said, you almost fell over just getting onto the bike.