People on nb are easily impressed, what is in the op isnt impressive at all, if anything actually the opposite. Anyway since you are making assumption, lets say the 3rd was to avoid doujutsu genjutsu. Itachi doesnt need his sharigan for genjutsu.
Secondly you speak of his lightning armour. If i remind you A's raiton armour in v2 is superior than his dad's and even that amaterasu got past his ration armour and started to hrut A for his to severe his arm.
Also the 3rd is slower than A, if i have to estimate id say his full speed is equivalent to A in v1. Sasuke who has an inferior speed and reflex ability was able to keep up with A in v1 susano. Ive little doubt itachi in his healthy state can accomplish the same, lets not forget that A needed v2 to evade amaterasu.
Now itachi has shown us the ability to show sighs of amaterasu, but then withhold and use it later. He did this against nagato. What im trying to tell you is that if the 3rd speed is roughly equivalent to A in v1 amaterasu can catch him. Add the fact that itachi can withhold amaterasu then fire it later it can get the third.
Going back to uchiha assumption to made. A if you go back to his fight with sasuke, looked him eyes, but due due to plot sasuke used amaterasu. If the 3rd was to repeat this feat and look into itachi's eyes no matter how hard his body his, tsukuyomi will mind rape him.
Next you speak of another assumption when you say the 3rd can penetrate ssuano, although nobody knows this ill give youi a closest assumption that it cannot. Itachi's susano alone in its early stages withstood kirin which is a far superior lightning technique than hell stab. If it withstood kirin, in its v1 stage, the idea of finger stab getting past a full susano hasnt crossed my mind. Add the fact that the 3rd has to come close to use that technique in a person in susano it isnt happening. Not only can a person in susano weave handseals to attack, susano itself has attacks in his physical prowess, yasakas magatama and in itachi's case the totsuka sword.
Look at it this way, itachi doesnt need to figure out the 3rd's weakness to defeat him.
3rd has to be stronger and smarter than itachi and he isnt in either case. Itachi takes the cake with mid to high difficulty.
Edit: From where im standing merely another pissed off minato fan :shrug: