After enough years in CX to qualify for a wheelchair...my vote is for frequency over heavy-lift.
The 380 doesn't lift cargo over 10 hours. The 777 does and provides the opportunity for frequency.
Given the CX spider network...stay with the 777ER as the long haul camel. The 330/350 as the feeder. Leave the freight to the 748 and add some PX to the fleet for destinations that are slot limited like LHR.
Ideal fleet profile in 2020:
A330/340/350 - 50
777ER/300/200 - 50
747PX 800 -10
747FX400/800-20
FWIW