I think if my bunnies had been bigger I would have split them into 2 pairs, as you say.
I agree with all your other points :thumb:
I think you also need to take into consideration the strength of the bond in the group. I was almost 100% confident that the Rexes' bond was stable and that splitting them up into two carriers wouldn't cause any issues. I also used to split them up for some vet visits and leave one or some at home. With Tui and Tethra I have also split them and for one of Tui's post-op appointments I just took Tui. As an owner I think you can judge how stable the bond is.