Same problem here… I spent a lot of effort on making things work on Buster, but failed.
I did a full-upgrade from Stretch to Buster, but some network-related services don’t work as they did on Stretch.
In my case, I am trying to make the ReSpeaker as an access point, but re-wifi.service doesn’t work on Buster(it fails with a kernel error). My guess for now is that some respeaker-specific packages(which are officially meant only for Stretch) don’t work with Buster, as can also be seen in the link below, which was in the repo list.
You said the upgrade completed without problem, but did the respeaker repo (deb [arch=armhf] http://respeaker.seeed.io/deb buster main) download successfully too?