Nagato has more power than itachi as nagato has a unique bloodline the Rinnegan which give him power to control all 5 nature elements plus he could use his six path of pains connected to him but itachi has sharingan whose speciality is to copy ninjutsu.genjutsu,taijutsu but against an opponent like nagato who uses kekke genkai(blood line techniques) sharingan is use less as it can't copy rinnegan 's techniques becoz rinnegan is a kekke genkai.now if itachi uses its all mangekeyo sharingan techniques such as susanoo. They will not work for the following reasons:
six paths are comrades and can break tsukoyomi , summoning will cause itachi to have to control it , thus releasing it , there is no proof that tsukoyomi even works on him.
now susanoo , its weakspot is at the base and on the back , shinra tensei is a ground attack controlling gravity , you have to be hella heavy to withsytand it , and itachi isnt what i would call a 5k ton monster , chibaku tensei worried itachi in the manga so whos to say it wont completely oblitorate him with full susanoo?
and amaterasu can easily be absorbed by nagato simples.
nagato also has the upper hand with a range of un copyable techniques and has every single chakra nature.
nagato curbstomps mid-high difficulty depending on knowledge