Considering there's only 1 in the game after 1 year and 3 months of LOI being added to the game and being nerfed already I think the sword should be as strong as it is right now. Call me biased all you want but pure wm RELIES on it's weapon, it has been reduced to a 1v1 melee pressing mastery, if end game swords (Which are next to impossible to farm rn) are being nerfed when there are waaay bigger issues in the game right now then I don't know what to say.
Fan auto on lvl 20 weapon = Free combo for elementals, FREE.
Earth being overly broken rn > 1 land of iron sword in the game that I use once a week.
I understand your point about it being a power creep and appreciate that you've tried to balance the weapon instead of just nerf, but this change will literally not even be noticed because there is only 1Yamazaru in the game and only affects 1v1 scenarios.
Nerfing said swords isn't going to change the fact that pure wm is just a melee mastery and people will always complain about THAT being broken, so maybe look to change the class instead of nerfing swords that take 1 year + to drop and over 5000 hours to farm an impossible boss.