The best translation for “Migatte no gokui”?

[Cipher]

While it's not alliterative, you could use 'zenith' for retaining the 'th' phoneme and layering the pun with 'zen' allusions, whether those of Zen Buddhism and concepts of awakening, or even the English colloquial associations with general tranquility, either of which would jive well with Ultra Instinct.

Just a thought.

I thought about that yesterday. Does "zenith of thoughtlessness" have near-alliterative punch?

I didn't quite think so, but maybe!

A shame, because "thoughtlessness" is a really good double-entendre in its similarity to the Japanese, working in both a literal description of the technique and some "selfishness" wordplay.* Again, not for the technique name, but an introductory descriptive gloss.

*Better than most other English options even, in its nuance match for 身勝手, which isn't "selfishness" in the gluttonous or narcissistic sense so much as it is the "doing whatever you want and not caring about the repercussions" one.

[baconisbacc]

For something that whittles the meaning down to a catchy name, how about "Flow of Zen," "Zen Flow," or "Zen Step?"

Zen Step slaps honestly