It's obvious Riot can't balance the thing. So just remove it. It's way to strong as is, and can't be put into a safe spot that doesn't make it insanely OP on anyone.
Or make it deal massively reduced damage to champions, and let it damage minions, turning it into a nice farm tool to those without one.
The problem isn't that Thunderlord's is broken, it's that its alternatives aren't good enough. That's how impactful a keystone mastery is actually supposed to be. They're supposed to be that strong, all of them.
Unfortunately, many keystones have an extremely unhealthy design that makes it so they can't be as powerful. Warlord's Bloodlust was disgusting when it was as powerful as Thunderlord's. So, either the idea of keystones being impactful is bad in general, or all other keystones need to be tweaked and brought up a little bit.
Thunderlords is quite balanced now, and although I commonly see it used, the alternatives have very clear purposes as well that can be much better than Thunderlords. For example, any tank running Thunderlords is missing out on a lot of tankiness from a Keystone mastery from the tank tree, and also had to go 18 into the cunning tree when they definitely shouldn't. The same can be said for ADC, Fervor can be exceptionally better on AS based champions although not always.
