mark h has given you excellent advice something we both agree on.
Unfortunately you appear to feel “that having a bigger range would be stupid for the moment”
I believe you are wrong. Think about your 7 batten slalom sail. The foot of the sail is cut very low and deep. This needs two battens below the boom. It generates power in an area of the sail that is easy to control and gives constant drive. Your down the line wave sail has a very high cut foot and only requires one batten below the boom. It does not generate the same usable power for a freeride board.
Without going into a heap of technical detail the actual power gap between your slalom and down the line wave sail in real terms will be greater than 0.7 when compared to two sails of the same design.
Buy the 5.2 monofilm freeride sail you will not regret it.
54kg sailors do not need to put durability at the top of their list.