I am personally a weight weenie but I am also the designated rider who'll carry our family's luggage.

We are Ortlieb all the way. We will ride with 110L of cargo space (2 rear on an extra wheel + front and rear on the bike proper) but I expect/hope that 80-90 will be enough. (we are used to tiny tents, have ultralight sleeping systems and prosper indefinitely on clothes that fit in 2x3L stuff sacks each. Add cookware, toiletries, food, tools and spares and that'll do.)

Even though we have (more than) what we need, I'll probably try kitty boxes. It seems to make absolute perfect sense.