Recent OS update 11.2.1 didn't fix this bug... Seems that my iMac switch from my router to the wifi repeater and back (Same SSID). If you have a repeater try unplugging it; connection is WAY worst, but no more dropping connection.

Found a solution using the Wi-Fi repeater (now with a different SSID) only for the iMac... Everything's back to normal. The absurd is that before Big Sur I never experienced those problems.