Given that it's generally local governments that are hesitant to upzone, is there any indication of some cities bucking the trend? What barred cities from ending single family zoning prior to these bills?
Now, you are effectively not allowed to do that. So the permitting process to build a duplex is as straightforward as the permitting process to build a SFH.
Someone else could explain better but I'm also fairly sure many of the new rules here (such as exempting CEQA) also apply to SFH. They just leave the current rules untouched for commercial/large-scale residential.
Under RHNA and the HAA, if cities fail to zone for their share of housing, many projects will be approved automatically and the city loses its discretionary review authority.