I understand his mentality that the kages aren't worthy of his full effort, but come on Itachi is about to cancel the edo-tensei and if I were him I would kill as many kages as possible before the edo-tensei ends
Right,
but he doesn't know about what Itachi is up to according to our knowledge unless Kabuto were to somehow inform him, and didn't Muu recently leave the scene of the fight of ET Madara vs. the 5 Kages. Thus, I don't think he knows anything about the potential threat to IWR posed by Itachi.
Recently for example, he used a Mokutoun clone, which he said he was previously the only to be able to distinguish because of his eyes (i.e. his 3-tomoe Sharingan, or MS/EMS [not sure if they improve visual discrimination since Itachi didn't note it]), when he could've used a simple shadow clone. He's screwing around with Hashirama's powers vs. when he started the fight and he came out as an Uchiha with his first move a massive fire release (the fire release: fireball jutsu is what defines an Uchiha as an adult if I recall Fugaku to Sasuke correctly). He then followed that up a combination of EMS (only Susanoo) and Rinnegan jutsu such as Shinra Tensei and the Kinjutsu to reign down to massive meteorites on the alliance forces. Ever since then it's been Mokutoun largely.
Now I don't think it's because he has some stupid man crush on Hashirama that he's spamming Mokutoun jutsu, but because he wants to master it or at the very least become proficient in the powers that he faced and found sufficiently difficult each and every time he fought Hashirama to use later on. Same thing every time you get something new and cool.
Still..., if he has Tsukiyomi and Ameratsu..., actually let's assume he does. Tsunade came straight on, and as noted was not terribly fast, possibly could've landed one of those two techniques on her, so why didn't he? I'm at a personal loss as to why if he really wants to kill the 5 Kage he doesn't use them or his 4th EMS technique. Honestly, right now they're set up for Tsukiyomi since he hasn't used genjutsu against them making them more likely to make eye contact, such as Tsunade did during her speech about the Will of Fire and the medical ninja rules. Didn't Tsunade just stand right up to him giving him a speech, making her a perfect target? As well, when they sealed his Mokutoun clone, they lowered their guard making some of them perfect targets for Ameratsu or any attack, but all he did was impale Tsunade with an attack she can likely repair from with Mitotic regeneration based on what we've seen her heal from against Orochimaru dicing her to pieces with the sword of Kusanagi, trying to get to Naruto pre-time-skip.
My best explanation atm... a combination of plot no jutsu and Madara's behavioral profile. ET Madara is holding back in order to prolong the fight,
and/or he just wants to win using his new powers since he already knows the limits of his other abilities (i.e. EMS and to a decent extent the Rinnegan) and that of his opponents, the 5 Kage. If he was fighting someone like Hashirama or of that level, I highly doubt he would hesitate to use whatever the appropriate jutsu and corresponding advantage it conferred in his arsenal to win whether it came from his EMS, Rinnegan, his enhanced Mokutoun via Kabuto, or ET regenerative abilities, etc... - quite considerable...
Peace
