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I am trying to convert filenames in russian zipfile to UTF-8 using convmv.
Original filename: "æ óá¡¿½∞¡δ¼ áα«¼áΓ«¼.jpg" (æ óá¡¿½∞¡δ¼ áα«¼áΓ«¼.jpg with slashes)

This analyzer (https://2cyr.com/decode/?lang=en) detected source encoding CP866 + displayed as CP437, and successfully decodes to desired С ванильным ароматом.jpg.

My question is, how can I set up convmv to decode it properly?
For convmv -f cp866 -t utf-8 filename, I get "already UTF-8", in --nosmart mode I get jibberish.

A single pass of convmv can only fix one level of wrong-ness at a time. Your particular file is more complicated. It was originally cp866, and it was at some point turned into UTF-8, but whatever did that conversion thought it was originally cp437 and so converted it wrong. To fix this, you need to run convmv twice:

convmv -f utf-8 -t cp437 --notest 'æ óá¡¿½∞¡δ¼ áα«¼áΓ«¼.jpg'
convmv -f cp866 -t utf-8 --notest $'x91 xa2xa0xadxa8xabxecxadxebxac xa0xe0xaexacxa0xe2xaexac.jpg'

Note that I had to escape the name in the second command. If you're running this in bulk and/or don't want to deal with doing that, you can use something like *.jpg, or put all of the affected files in their own directory and use the -r option to convmv.