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Per Aruch Hashulchan Siman 95:5 in the name of the Elya Rabbah it is proper that the feet should remain together after Kedusha until Hakel Hakodesh.
Rav Shlomo Zalmen Auerbach Zatzal (Halichos Shlomo, Tefilah, Perek 8: Ha'arah 60) maintains that on Rosh Hashana and Yom Kippur when there is a long time between Kedusha and HaKel Hakadosh, it isn't necessary to stand until HaKel Hakadosh
Tefillo Kehilchoso says that the sense of the majority of authorities is that one can move after Yimloch. He quotes (in 117) MB 95 . The reason given there not to move the feet in kedusho is because we say “keshaim shemakdishim” (as the angels sanctify). We are to learn that the angels only say up to yimloch. We stand with feet together imitating the angels.
He further says, “but 125  MB” quotes sources that say one cannot speak until after Hokail Hakodosh (presumably such speech as is allowed in the repetition of the Amidah).
He quotes the Eliya Rabbo (95, 7) and the Kaf HaChaim (16) as saying you must wait until after Hokail Hakodosh.
That Yimloch is part of the praise of the angels is referred to in this blog who gives midrashic sources (see for example Psikta Rabbosi 20).
I have heard that some say it depends what text of the siddur you're using.
In Nusach Ashkenaz, the chazzan concludes Kedusha with L'dor vador nagid gadlecha, "and we shall continue to sanctify You from generation to generation ..."; this is a direct flow from the responsive kedusha, which wouldn't finish until the chazzan completes his blessing of hakeil hakadosh. Thus, people should remain in place until that point.
In Nusach Sefard, the chazzan simply continues ata kadosh v'shimcha kadosh, which is the standard blessing without kedusha. This means that kedusha has finished, and he's starting a new thing. If so, the last line of kedusha is yimloch, and people can move about as soon as the chazzan begins ata kadosh.
Note that a community where people remain at attention longer should probably continue that practice, rather than encourage anything with might reduce proper synagogue decorum.