`apt upgrade` package without marking as manually installed?

By poking at the sources, I found the feature you are looking for, but it was made available only in a commit made a few months ago, so is available only in bullseye (future Debian 11):

Support marking all newly installed packages as automatically installed

Add option '--mark-auto' to 'apt install' that marks all newly installed packages as automatically installed.

The equivalent configuration option (having no effect in Debian 10) is APT::Get::Mark-Auto.