Paul, I own both the Mamiya 6 and the Bronica ETRSi. The Bronica did not become redundant when I got the Mamiya 6, far from it. Both cameras do complement each other beautifully.
With the Mamiya 6 I gained a more portable camera, lighter weight, and above all, silent picture taking. This said, it is very slow to focus, and it can never beat the speed of use of the Bronica (mine is coupled with a metered prism and motor winder). I never suffered the lack of a faster lens on the Mamiya 6, I can handhold it down to 1/8s with the 50mm on, without problem.
Also, I regret that the focusing distance is so long that I can't really do portraits. Or when I do, I feel I am constrained by the focusing distance of the lens.
As to the retractability of the lens on the Mamiya 6. I would not put it down as a decision factor as you do. Yes it is nice. I personally never retract it to preserve the bellow. Those babies can't be easily serviced, if at all, they need TLC.
A nice travel and compact rangefinder that you might want to consider is the fuji GA645Zi. It is compact, very lightweight, autofocus and its lense is very crisp. I own this one as well. It is not very convenient to use in manual mode, but it is dead fast to use, and the lense is very sharp. Every time I enlarge a picture taken with it, I am amazed by the crispiness I get from this small glass.
Good luck.
Delphine