I feel like this creates weird incentives where you either want to wait out Rage so you cast a 'fresh' one that won't debuff you, OR you go so hardcore for recharge that the time with double-rage is worth more than the 10 seconds of exhaustion. If someone's got 10 seconds of double-stacked Rage, in their build, it might not even be worth casting since you're also getting 10 seconds of complete weakness in exchange for 10 seconds of moderate improvement.
Here's my suggestion: If you cast Rage while Rage is already going, then it resets the duration of all stacks of Rage currently going. So yes, if you can stack Rage at all, then you can stack Rage as many times as you want (up to the damage cap and beyond). However, each stack of Rage beyond the first also applies the defensive debuffs that you guys added:
-10% defense irresistible
-20% resist irresistible
-20% elusiveness irresistible
And of course, these would stack with themselves. So if you want to get 4 stacks of Rage and decimate everything with your 320% bonus damage, go ahead! Just watch out for that -30% Defense and -60% Resistance, that's gonna sting. The idea here would be that one stack of Rage is fine, but after that you're being reckless and endangering yourself for more damage. I think it really drives home the thematic of Rage while also adding a new gameplay dynamic - choosing whether to double-down and continually accumulate more damage and vulnerability, or letting it fall off and reset the 'cycle' of building Rage stacks.