Complete the Malta carriers or refit older ships?

Would Britain have been better completing four Malta-class carriers while retaining Victorious for trials and training?

Probably. Completing the Malta class and retiring the other armored carriers was likely a better route than trying to keep the mixed older force working.

The choice is a fleet-level investment decision. Completing the Malta class could have produced a more coherent carrier force. Repeated attempts to sustain or rebuild older armored carriers consumed money and time. The same tradeoff appears in the wider argument for standardization and common logistics.