I considered the Centaflex but I would like to fit really soft engine mounts
so preferred a double CV coupling.

Centaflex AGM is a double coupling - one at the gearbox flange and one
at the thrust bearing. CV joints in Aqua and Python are still metal to
metal - and transmit noise. "Really soft engine mounts" - mounts
should be calculated to the weight they have to support. My engine has
6 mounts and each were calculated to take account of the static weight
plus the dynamic weight increase and or decrease caused by the torque
of the engine. The rubber is still quite stiff. Really soft mounts
may not be designed for the weight and will just collapse/bottom out
and you will have a very healthy noise short from engine to bearer..