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.