Arelith

Presently, Thundering Rage still retains the 2d6 Massive Crit. effect applied onto weapon, like vanilla. Also like vanilla, this does not stack with Massive Criticals that are already on the weapon - And it DOES NOT override it, even if the weapons massive crit is lower.
https://imgur.com/a/mYxGnY7

This means that to fully make use of Thundering Rage, you'll need to use a weapon without the property innately on it, and I'm aware there are FEW options for this, locked behind Racial/Class UMD usually. Even weapons that appear to be 'Barbarian' conflict with Thundering Rages boost. And yeah, 51 weapons with Massive Critical.
https://imgur.com/a/ftQ4ZYO

The suggestion? To change the Thundering Rage damage application to its own separate property. I will first say that I would wish this to be a WEAKER effect than the 2d6, as to not have any large damages stacking with one another. Like a weaker version of Overwhelming Critical, doing 2d4 or 1d8 instead of 2d6. Obviously not multiplying with crits.

Presently, it almost feels a waste using many of the unique +4s with Thundering Rage, as you're cancelling out a property of the Rage feat entirely.