Sequence of steps and bridge network configuration on native Debian Buster 10.3 host seemed to me a bit different from manuals which are available in meantime on the Net. Specifically I’ve undertaken some additional steps to fix error with Radeon kernel modesetting enabling also configuration bridge to physical LAN is supposed to be done in the way different from how it works on LXDE 4.

Source: LXer – Just another KVM setup on Debian Buster 10.3