Reading some of the things here, (not all of it, so stuff may have been said already). I will just input my thoughts:
Sukuna holding back isn't because he wasn't going all per say as this is still true to an extent (Instant second life regen), its because he couldn't. Black Box, Flame arrow, Lightning dagger, none of them would have worked on Gojo, its more so that Gojo isn't able to experience all because of his cursed technique blocking everything, for the most part. This obviously required Sukuna to use Mahoraga to create a blueprint of Gojo's infinity, making it the target, something he wouldn't have wouldn't without him. This meant Sukuna could target the very space, world and existence of Gojo and his "universe", so by default regardless of durability or block, Gojo would be cut by extension of his place in that "universe". The whole 20% thing doesn't literally mean that characters are only 20% at strength (which I think some of you may think), its that overall without the Technique sorcerers are left with an overall strength of 20% (reinforcement and basic martial arts), overall they aren't able to fight at 20% of their strength (Strength= abilities and arsenal).