You don't need to forsake group updates, just do them differently
TL;DR, you should look at what makes the well-received updates to be considered successful by the players. I would say that most people want champions to be familiar, especially the mains, and will only (sometimes not even then) accept a radical departure if it's packaged in a complete VGU. Take what that champion already does, and then make that feel better, make it feel like a natural evolution of the champion, and you'd have way more successful reworks.
And by that I mean, don't try to rework champions to fit a new mold, or to fit a new group of champions, but improve on what's already there.
Look at the reworks that are considered the most successful in this scope; the small-to-medium updates that are purely kit changes.
for example. Some of these were standalone changes but they're in roughly the same scope. What do they have in common? They aren't radical shifts; they take what you have with the champion already and try to play that up.
Darius was a monster lane bully who wanted to teamfight because of his reset capabilities but dropped off like a rock. Reworked, he has more opportunities to survive being collapsed on and can more consistently fulfill that teamfighting beast fantasy, and his lane has far more counterplay as a tradeoff. Katarina has been emphasized as the AOE chain-killing monster she always was, but now she's better at it; more slippery, and much higher potential damage, but now it's telegraphed. Vlad is still the blood-sucking hypercarry, but he's got two build-up attacks that gives him and his opponent more to play around than Vlad passively spamming Q and E and eventually winning by attrition. Ashe received a change to her Q and her Focus passive to give her more to do and proper windows of power. Shyvana gained a new E in dragon form that gives her some area control and a couple of buffs. Brand is still the mage you pick for dominating teamfights through raw AOE potential, but now he trades up-front damage for the stack-and-pop explosion of his passive, which in turn gives him the ability to chain MORE AOE.
These guys didn't change radically; they might play a little or a lot different, but their goals, their purpose is still the same, and the character is still intact.
On the other hand, the reworks that tend to draw the heaviest criticism are the ones that have a massive departure from the original champion, or if an uncomfortable gimmick is placed in their kit from out of left field.
fit in this category. They either feel too different from the source material or their issues weren't addressed.
Mordekaiser gained a few gimmicks, and not a lot of his issues got solved as well as the whole 'making him a bot laner' thing that proved disastrous when he was strong enough to be meta. Originally, Malzahar's kit had Voidlings as a passive, a compliment to the rest of his kit, and he could opt into AD builds if he wanted to focus on them as his damage. After the rework, they're now a normal ability and are core to his functioning, as well as gaining a passive Banshee's shield that makes balancing him a struggle. Skarner's spires were impossible to make work; he was either too strong in them and terrible outside, or he could be made good overall with spires being meaningless. The lack of control around them makes capturing them feel like a chore, because he essentially brings Dominion to the Rift. Garen is a funny exception in the sea of reworks because he didn't change enough; where none of his issues as a champion were solved (binary, feast/famine gameplay and very little to do overall), he was also given an unreliable steroid that probably made him harder to balance and overall left him in a worse spot than he started. Rengar went from an assassin who had a focus on setting up his stacks before he went for a kill (like a proper hunter) to a bruiser who can only stack up in the middle of a fight. Graves turned into a tanky-assassin-DPS jungler and I know a couple ex-Graves-ADC mains who are still salty about it. He's not unsuccessful, but he's pretty controversial, so I felt the need to list him.
I'd like to make a separate paragraph for Lamb/Wolf, because I both appreciated the champion and recognized that she was terribly designed. Before the rework, I think Kindred was actually fairly unique. As an on-hit marksman/skirmisher, she had a particular problem with being reliant on marks to be of any value. The rework.. didn't really fix this; by shifting her to crit builds and putting her focus on late game, she's now reliant on marks and gold to function, but her base stats are too low for her to reliably fight enemy junglers for marks early on. Marksman-tier stats simply don't work in the jungle, especially on a champion who's expected to be ganking and counterjungling to power themselves up. They're starting to improve statistically but I think that the main goal of the rework, less feast and famine, didn't succeed, and a lot of Kindred mains seem to dislike the emphasized change on her build, too.
Group updates are completely fine in my opinion, but I think the biggest pitfall Riot has had is trying to do group reworks that tie them to an overall 'theme.' I think it's most noticeable with the assassin update, because they added a lot of delays and funky in-combat setups on champions who simply won't work as assassins with that delayed damage. You can kind of see it in the tank update too; they tried to make all 3 of the reworks aggressive CC bots, with a focus on initiating fights and constantly applying hard crowd control, and some of it at the expense of other strengths they used to have prior. As a result, I think the whole 'making them niche' thing falls flat, because their differences now boils down to how they apply their CC. This isn't really as bad with the Juggernaut/Mage updates because they were earlier and a lot less specific, but I think this was one of the better parts about them; for the most part there weren't any radical departures.
Keep this in mind when/if you decide that you'd actually like to do a group update on Divers, Riot. You don't have to force change; while many of these diver champions are indeed very samey, most or all have unique qualities that you just need to expand upon more to diversify the roster and separate them from each other. Even a big change from an original function can be well-received as long as the champion at least feels like the same person, but improved. So I think, if you go through with it, you should focus less on an over-arcing goal and rework them as individual changes that just happen to be released simultaneously.
it totaly work
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