Only do delves solo. In fact with the changes to the dungeon weekly counting heroics I doubt I’ll ever do non-queuable content again. The only bad thing about delves is the miniscule crest rewards and the lack of gilded crests from higher tier delves. I haven’t seen any reason to do beyond T8, although the 11.1 changes may push me too, assuming the amount of crests increases to a reasonable number (should be 5 like a failed M+ run). 
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Organised group > solo >>>> pugs.
I’ve only ever done Delves solo.
On the ptr, the runed crest drops have been buffed significantly in the higher tiers. They’ll definitely be worth doing.
I prefer the solo gameplay. Then, if mistakes are made, I am the only one affected and do not feel as though I need to apologize to others.