[name_u]Micky[/name_u], [name_u]Micah[/name_u] and [name_u]Milo[/name_u] are some of my faves - maybe [name_u]Cal[/name_u] or [name_u]Mel[/name_u] might work too
Also you could come up with something like [name_u]Tiger[/name_u] or [name_m]Bear[/name_m] that has nothing to do with [name_u]Michael[/name_u] & end up calling him that.
I think [name_u]Micah[/name_u] could be used as a standalone name as an honor for [name_u]Michael[/name_u]. Maybe that’s the better choice if you don’t really like [name_u]Michael[/name_u] all that much?
Also my cousin’s kid is [name_u]Michael[/name_u] the fourth but goes exclusively but a completely unrelated nickname that they liked. So that could be an option as well.
[name_u]Micah[/name_u] and [name_u]Misha[/name_u] are my 2 favorites, and I think they have the warm feeling you’re looking for! But [name_m]Mac[/name_m] is super cute as well!
[name_f]My[/name_f] grandma had a friend whose son-in-law was an american naval officer named [name_u]Michael[/name_u]. Se wasn’t able to pronounce his name correctly, so he was just “Mico” (mee-coh) for her.
I second [name_u]Micah[/name_u] and [name_u]Misha[/name_u], but I think even [name_m]Ike[/name_m], Key and [name_u]Kai[/name_u] could work.