Oh wow… Mate… You’re way overestimating the power of group play.
There’s wudi and raxx (linked above) that did exactly that, cleared 150 playing only solo (and on harcore). They do play many hours per day, but their monk 150 clears were withing a couple days, so well below 50 hours playtime total. Dieoxide above also did all the rank 1 clears. Don’t think he was technically 100% solo only, but if you’ve watched his stream, he is usually getting less XP/h when in a group, because he is nice and playing with ppl below his level, and almost always with random group compositions very far from meta runs (“sure, join in”), so it’d be quite unfair to accuse him of OP META RUNS!
The streamers are obviously mechanically very skilled though. I am an example myself otherwise, kindof tryhard, but not with the skills of the streamers. I’ve played maybe 3hours per day average last two season. Plenty of time for work and life outside D3. 100% legit, no thud, no bots, no macros. Some in groups, but a lot solo, especially S25 I found it hard to find groups, and towards the end of S24. I reached 3900 and 3600 paragon I think. I soloed GR148 S24 with firebirds (got a 15:03 finish at solo GR150, but no cigar… 
), and of course soloed GR150 on innas S25 like everyone else. Also got a rank 1 GR145 clear on a minor set leaderboard in the second half of the season.
I’ve been doing both, so let me break down the difference between solo and group.
- XP/h. But last two season, we’ve had builds that are quite strong in solo XP as well: mirror image fire birds and fire innas, so I’d say I got around 80% of the XP solo as what I’d get in a group with ppl at my level. Both builds soloed 130-135 range in 3-5min average, innas a bit faster than mirrors. It’s nice solo, because you never have to wait for others, you don’t lose time trying to find players, etc, so I could decrease town time a fair bit solo, which increases XP/h. If I had played only SSF, I would’ve lost maybe 20% of my XP, which translates into 7% less paragon points (paragon scales with cube of total XP above 2250p), so I would’ve been maybe 300 paragon points behind. This translates into 1500 main stat, which is around 5% damage for me with ~30k main stat. 5% damage is less than a third of a GR level.
- equipped gem ups. Playing gems could be level to 150 solo this S25 easy, so S25 not a difference. S24 not quite the case, I’d probably be playing with level 145 gems or maybe slightly higher. Even with 140 gems, you’d get a 57% damage bonus from bane of the trapped instead of 60%, which is a 1 - 1.57/1.6 2% damage decrease. With three damage gems (the effect is smaller on stricken, and no effect on enforcer with mirrors, but lets ignore that), it is a 6% dps loss total. Again, a third of a GR level.
- augment gems. I’d do around 141 augments probably. Both innas and mirrors could solo farm GR140 comfortably. So that’d be 9 times 13 items lots (although rings and amulet may not be ancient), 117 levels, which means losing just below 600 main stat, meaning another 2% or so of damage, 0.15 GR levels I think?
Putting everything together, had I played solo only, I would’ve lost at most 1 GR level S24 and S25, probably less. Which would’ve been GR147 S24, still GR150 S25, and I’d have a rank 1 GR144 on minor set leaderboard. And I’d have 3600 and 3300 paragon points instead.
I think the most constructive way to continue this discussion is for you to tell us what builds you’re running, share your profile with your gear, and maybe we can help you clear higher more easily. I don’t know much about what you’re playing or how much, but we might be able to suggest some changes that’ll allow you to enjoy the game more.
