I have been having issues with wifi on my Mac Mini (2018) since upgrading to Big Sur. It would connect for a short period, start to be unreliable, drop connection for about a minute or so and then be unable to find the network for a while. After trying all sorts, including wiping and reinstalling Big Sur, I traced the issue to either the connected old Apple Cinema HD display or the (generic) USB-C to DVI adapter I was using for it. Since unplugging them my wifi issues have disappeared and everything is stable. So certainly worth checking any adapters you might have connected.