Honestly, I didn't have to rerun much of anything prior to 120. My last run I didn't repeat any content at all, not even a mephisto repeat, until after 115. Some extra Tran Athulua runs at like 118, that was it. I was able to do everything once and clear all maps up to Baal's entry portal (and the easy/early ubers once) to just about hit 120 cleanly. To me, that's no slog but intended progression of the early game.
You must be skipping/postponing areas in the natural progression to experience that? I realize my way may be atypical, but so much of that content can only be properly experienced once. Rather that personally than beginning the repetitive grinding for XP in late nightmare on. Never understood the attitude of skipping as much vanilla zone progression as possible, at least when the content is engaging from a balance standpoint.
Unless you mean the act to act or area to area progression becomes too difficult at that stage? I'm genuinely curious as to what your early game progression looks like for 105-115 to bother you.
I only felt the repetition slog become unduly tedious at 132 until Dimensional Labyrinth became practical. Fauztinville is dull these days. Viz Jun is fun, but falls off earlier and the loot is less desirable on average. Bremmtown helps diversify the play experience at least and kicks out XP nicely for how quick the map is.
I do agree the new K3K is boring and imo unimpressive in loot. Signets seem faster in the Jungle. Sets aren't really target farmable in K3K anymore without trading. It just doesn't have anything going for it. Why go there beyond the charm? I used to love that place. Seriously. I found solving the totem's for various builds fantastic in tactical variance. Yeah, it sucked hard for the occasional build. The joy of 90% of builds overcame those outlier experiences. Only uber change I am heartbroken over...
Sunless being simple seems right however. It is the introduction to rift ubers. It aught to be accessible and straight forward. Plus, its way less obnoxious than the old one. Anyone else have nightmares trying to find the charms under the 1000000000000000 individual gold piles? That made snorting asbestos seem healthy. If Sunless were made interesting then something else would have to (or simply should) functionally replace its role in the new player learning curve.
The High Heavens are under-tuned now imo, but conceptually I find it fine. Just could use a little more threat from the halberd guys or an uncommon supporting enemy type mixed in to liven it up slightly. Just needs a little something to spice it up again. Used to be a scary place for some builds anyway... which was a good thing, no?
I really am curious what early game progression looks like for you. I am entirely with you on early rifts having become downright sterile. I just question how critical repeating early rifts actually is when walking through the game semi-throughly just once does the trick in this level window.