I don't think addons really address the difficulty curve issue at all. I think there's just a pretty severe jump between P1S/P2S and P3S in terms of what is being asked of players.

Endsinger EX kiiiind of fits between 1/2 and 3 in terms of difficulty imo, but even then it's a still a severe jump. I wish P2S was a little harder, maybe with tougher heal checks and more for the offtank to do, then maybe the jump to p3s wouldn't feel so jarring.