so imagine what deva pain can do to the 6 tailed beasts
*chuckle* It doesn't work this way.
Kakashi has simply gotten better.
He held his own alone against 4 edo jinchuuriki in V2 with Sharingan and Rinnegan.
Of course V2 that Deva fought was stronger than any of these jins. Or I'd say it would be stronger if we were talking about normal jins, but these:
-are immortal
-have a shared vision (Rinnegan)
-have increased perception and dodging abilities (Sharingan)
Plus, Kakashi faced
4 of them at the same time
after already having fought for an entire day elsewhere, whereas Orochimaru (albeit handicapped, true) could not handle one mortal V2 jin without Rinnegan or Sharingan.
There is literally nothing to suggest current Kakashi would not be able to keep up or defeat 6-tailed Naruto, even without resorting to Kamui.
Plot - he should have just use Kamui to take his head off from the start. He was right next to him when he stopped Deva Path killing Iruka
Plot is NEVER an excuse. Even if plot was the "outside" reason for Kakashi not using Kamui back then, there must always be an in-universe reason as well. If there's not, we take the most plausible explanation.
In this case there are two plausible explanations I can think of, though they overlap:
-He still didn't have his Kamui perfected back then
-He knew there were 6 bodies and that he couldn't possibly kamui all of them (though it would mean as well he didn't have his Kamui perfected yet, since current Kakashi would be capable of using Kamui 6 times with ease) so he wanted to try and handle as much as he could without MS