What fukasaku said is in regards to basic ninjutsu and extra but what you're saying is a bit contradicting because you're agreeing with me about itachi using more mental energy jutsus and disagreeing with me in terms of the balance thing.No, read below.
No, read what fukasaku said, "you already know how to balance mental / spirit energies to create chakra for Ninjutsu". He's clearly talking about ninjutsu, not senjutsu (which he adds later).
Regarding what shikamaru says:
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As with genjutsu, the doki Tayuya summons and their worms are based on Yin release (and Shikamaru hints that, since he says they don't have substance). Yin is only one type of chakra category, which would explain as a matter of fact why Itachi
was good with Genjutsu and why all uchihas can cast genjutsu with Sharingan, but that in turn means he cannot use the other types (katon, the dojutsus, etc) as efficiently, since the chakra ratio is not leaned towards spiritual energy (katon might be an exception, since it may be possible that it was his chakra element affinity). Basically what we know is that all nature transformations share spiritual / physical energies in balanced parts, Yin (like the Nara's clan shadow techniques) uses more spiritual and Yang (Like Akimichi's size changing techniques) uses more physical.
So basically what I mean is that Itachi's reserves, distributed as they are (and taking into account that Uchiha inhereted spiritual energy from SO6P), are geared towards a more efficient and prolonged use of Yin release, which doesn't mean he had vast reserves to use the other chakra manipulations as efficiently as this one.
Well, this got interesting all of a sudden, I've never paid too much attention to the chakra natures and stuff other than the basic 5 nature transformations.
Shikamaru said that it is a "semi substance" that it's a chakra with an appetite. You can't have chakra without both physical and mental energy, but the thing lacked enough physical energy. He's saying that regardless of what it is, it's still chakra a ninjutsu but it only lacks enough physical energy to have a physical form.
The Nara clans shadow possession does not have a physical form, correct? That means it comprises of mostly mental energy than it does physical energy similar to like Tayuya's jutsu, right?
So you are correct that Itachi geared towards jutsus that had more mental energy than physical energy but in the end of the day it is still a jutsu and you can not use ninjutsu/genjutsu without chakra and you cannot use chakra without both mental energy and physical energy regardless of which one is greater than the other.
And in no way did I said that Itachi's chakra is vast but it's sure no low either.