Uhm, imho there are some details worth delving into.
- paragon XP difference between solo and party doesn't actually matter a lot. Yes, you need paragon for leaderboards but since who plays in party gets more exp and can run also solo, there is no way to counter it. Either block "party characters" from solo leaderboard or the opposite, make people choice at season start if they want a "true solo char" with dedicated leaderbards. That would solve most of the issues.
- related to point above; there won't EVER be way for casual to compete against hardcore farmers. You either play uber-efficient from time 0 forever or you're left behind. There is no point in discussing this.
- again, what's broken is the synergies and not builds themselves; single class wise, it's been dealt with. Party wise, support is what breaks things.
- normal rifts are useless at this points. As you said, you need them for keys as both xp and drops are more consistent in GRs (plus the advantage of much more difficulty granularity making every minutie of your time potentially worth more).
- again on paragon: the point is not farming forever. It's what everyone wants. The point is WHY farming forever. Having something mandatory for a small part of the playerbase while the majority simply has no use in that and the feature becomes pretty much passive bonuses you get for doing other stuff isn't what i would call a good design.