The update and upgrade went fine, but full-upgrade fails due to the following error message:

The following packages have unmet dependencies:
 libc6-dev : Breaks: libgcc-8-dev (< 8.4.0-2~) but 8.3.0-6 is to be installed
E: Error, pkgProblemResolver::Resolve generated breaks, this may be caused by held packages.

From what I see on the, Debian testing should have libgcc-8-dev: 8.4.0-4, so I don’t see why an older version is to be installed.

How can I fix this, to finalize the bullseye full-upgrade?

Installing gcc-8-base (sudo apt install gcc-8-base) appeared to do the trick for me and fix the problem for me.