Nex has essentially been meta to solo before even EGWD existed, let alone necromancy which has trivialized it further.
I'm sorry, but calling Raksha 'arguably way easier' then Nex is extremely disconnected, and soloing Nex is like turning your brain off entirely in comparison.
The only people that might consider raksha easier are those that have a good amount of PvM knowledge and utilize optimal consumables, gear + rotations - and they would think so because with enough DPS, you can skip virtually all of Raksha's mechanics. This, however, is not the norm for the average player that dabbles a little in PvM.
Nex, on the other hand, is quite literally "just soul split and hit the boss" with the one exception of the ice prison attack, which is primarily annoying more than anything else.
I disagree with the notion that the timer CA's being solo-dependent isn't indicative of a boss' considered status. It is generally consistent that on solo bosses where you can play as a group, the timer needs to be done solo, however the criteria for the mechanical CA's are individualized - that's to say, someone might complete a CA to not get hit by X or 'don't move during', whilst another player might fail it during the same kill. The successful player will get the CA, and the one that failed will not. For Group bosses, it is generally indicated that everyone must fulfill the criteria to get the achievement. Either all get it, or no one does.
As for bosses going from group to solo and not the other way around is as obvious as breathing. To go the other way would require bosses to be updated to be significantly more difficult than they previously were, and this is not a practice that is generally done, not only in RuneScape but many other MMO's and games in general. It is rather unusual for devs to actually buff bosses after the fact, unless there is an extremely good reason to do so. If they launch under tuned, it is very often "ah well, we'll try to do better next time, we'd piss off the majority if we go and make it more difficult now".
I do not think that the concept I've proposed for Nex can be applied to a lot of bosses. The only other boss in a similar situation is Kalphite King and his 'green' mechanic, although I can't speak to that, since I've never gotten around to doing much of that boss. I do have plenty of experience with every other mid+ boss in the game though.
The main thing other older bosses would draw upon from sanctum would be the very clear telegraphing of where and when abilities hit, but I don't think that really sits in the same boat as the actual functionality of a mechanic. Bit of a "Form vs Function" comparison.