So you want drums to be mandatory anyway, and the nonsense you said about them not being mandatory, was a lie.
This will 100% still have leatherworking be mandatory for top guilds, they just stack for the effect.
If you can still roll them, back to back to back and have 100% uptime, then the change you implemented is pointless.
There are only 3 viable solutions
Remove the cooldown so 1 person can keep up the buff per group, 5/25 players with LW.
Have the buff raid wide so 4 people per raid can have it rolling, 4/25 players have LW.
Add tinnitus so that it nerfs them and you can only use them as a short cooldown, 5/25 players have LW.
The proposed change you are implementing still has 20/25 players Leather Working.