Delves were in a fine state before. God forbid people who grouped up found them easier, but you decided to mess with them so much they are totally busted in many ways now.
Just revert it back to the way it was. 8s were “okay” solo, easier with a group, but overall they were more FUN in both cases. Grouping was much faster but you still couldn’t ignore mechanics.
If you want people to engage with your content that is now sucking up hours and hours of dev time, why not just go back to the point where your playerbase found them fun? Isn’t that the goal of a video game after all? To have fun? The ilvl of the rewards isn’t even THAT big of a deal, so if you’re doing all this to “curb” ilvl gain vs difficulty, just forget it. You’re ruining the experience for EVERYONE trying to smash this with a hammer over and over to “fix” things.
Someone having trouble soloing 8+ could group up and actually get their bountiful delves completed AND have fun doing so because they weren’t slogging through it or having to play perfectly because they play a spec that lacks the utility/tools to engage with some encounters appropriately. I really think you might be in over your head with this balancing nightmare of trying to balance 12(more counting the different stories) “mini-dungeons” for each and every role and spec in the game, so you may as well just open it up and make it relatively easy for everyone by going back to the way it was.