This approach is based on filbranden's git-rpmbuild and his talk during ASG2019.
git clone https://github.com/systemd/systemd fedpkg clone systemd fedora-systemd cd systemd rpmbuild -bb --build-in-place --noprep --define "_sourcedir $PWD/../fedora-systemd" --define "_rpmdir $PWD/rpms" --with inplace ../systemd.spec sudo dnf upgrade --setopt install_weak_deps=False rpms/*/*.rpm
--without lto and --without tests may be useful to speed up the build.