In no way was Obito scared of Itachi and here's the biggest reason.
Obito had akatsuki under his control and if he so chose, he could have commanded them to eliminate Itachi for him if he posed a threat. And there's no way Itachi could solo Akatsuki.
Obito stated that Sasuke's power will surpass Itachi's....so why would he risk giving Sasuke the EMS if he is already afraid of Itachi? Not to mention he already threatened to kill Sasuke if he doesn't obey which means he's unphased by their power.
They made a pact when they first met. Why would Obito be afraid of Itachi if they just only met and knew nothing of his powers? He could have just attacked him and tried to end his life, but he didn't. He just came to honor his agreement in exchange for help get revenge against the Uchiha.
He would be more afraid of Nagato's powers than anyone elses
He still had the powers of mokuton and the sharingan. Even as a kid, he did more damage than most characters and experienced more terror.
Obito could still control the Gedo with just the sharingan and Hashi cells. He knew the kinjutsu of the Uchiha along with other techs which Itachi had no knowledge of.
Itachi states that "Madara" meaning Tobi, was his mentor and accomplice.
Obito fought Minato when Obito was still a kid.....meaning he's pretty fearless to do such a thing, especially so young. And if he's not scared of Minato, then he's not scared of Itachi either.
He already met the most terrifying force in the Naruto, Madara Uchiha, so why would he be afraid of someone like Itachi?