But as the 9-car Voyagers are already at capacity
9 coach Voyagers are nowhere near capacity in my experience. What however does frequently happen is that if the A - F section of the train is nearer the entrance at major stations, that section will become wedged as that is a) a shorter walk b) where the reservations are. G - L will however be far quieter. I've travelled on trains from Bristol to Exeter where there was standing room only in A - F but I was sharing coach L with 2 other people.
Just about every service I get on - at weekends mainly - has one of the Train Managers announcing that G - L is far quieter than A - F if anyone wants to move there at the next station. It's possible weekday commuters are a bit savvier.
XC should definitely have (in any reasonable world) fixed 7 coach formations on the classic routes when the Voyagers are replaced. Two trains joined together leads to the situation described above where passengers are distributed unevenly, and also the other recent ridiculous situation where all the passengers are wedged in one unit whilst the second is literally lugging around fresh air as it's locked out of use because of staff shortages.
9 coaches would be nice but is exceedingly optimistic. You'd also need SDO or the last 2 carriages would have to be locked out of use south of Exeter.
With 7 coaches, granted this may be overkill at the ends of the route, but it would be extremely valuable in the core, require fewer staff than Voyagers for more seats and no time or personnel cost for attaching / detaching movements.
For all the weird and wonderful places you could join with XC, call me old fashioned but I'd rather they focused on providing an adequate passenger experience on the routes they currently run before the Voyagers start running ff the beaten track.