He didn't use it vs Obito because lets be honesty...Kishi threw that in there for Minato, but it makes sense to make him not good at sage mode.Minato can only outlast with full intel. only perception sensory can properly counter visual genjutsu because they can still defend without their eyes open unlike other sensory according to jiraiya. without intel, it's unlikely minato would go straight sage mode unless he felt he needed to considering he didn't use it against obito. minato can scatter his tag kunai's to counter genjutsu but susanoo is in the way.
Him and J-man are normal strong ninjas without bs eye powers or sage abilities, etc.
Minato does have knowledge on sharingan lol... I mean it's the sharingan Minato isn't a idiot he's actually smarter and faster than Itachi.
He figured out how to defeat kamui in 4 seconds IN BATTLE...figured out COAT... the creation of all times and how do nullify it IN BATTLE while 8 Gates Guy and Juubi jin Madara were fighting at insane speeds.
Minato is an in battle genius let alone his ingeniousness in itself. To say he doesn't have basic knowledge to fight an Uchiha is insulting as a ninja and as a hokage. He was Kakashi's teacher for god sake lmao he grew up with Uchiha lmao so insulting and kind of retarded of you people to assume the youngest hokage not to have that knowledge.
Itachi has tskuyomi, right he knows nothing about that, but Minato has better reflexes, reaction time, and is a sensor who has ma and pa for sage mode.
He can tell and feel the difference of the chakra building in his eye. Being that didn't Itachi lose to Shisui many times...just saying even though he has sharingan too you can't deny the fact that he is well known for his speed blitz shunshin (which I'm sure played a big role in Itachi's many losses to Shisui)
Just throwing that out there. Itachi has no knowledge on Minato as well lets not forget that at all. Like completely no knowledge.