Not sure what a Rando bag is despite done my first brevet in the 80's
I like the tailfin aeropack a lot. I also like the Swift Paloma a lot....check it out. Ortlieb makes a pretty big seat bag that is waterproof, I used that one PBP and on a 1200 this year. Those three cover any combination needed on a brevet. I do not like frame bags or panniers while randonneuring. The big seat bags used backpacking are also seen a lot these days, they are too much for a brevet and would not recommend that for rando bag