  CeCu$_2$Si$_2$ is an exemplary correlated electron metal that features two
domes of unconventional superconductivity in its temperature-pressure phase
diagram. The first dome surrounds an antiferromagnetic quantum critical point,
whereas the more exotic second dome may span the termination point of a line of
$f$-electron valence transitions. This behavior has received intense interest,
but what has been missing are ways to access the high pressure behavior under
milder conditions. Here we study Si $\rightarrow$ P chemical substitution,
which compresses the unit cell volume but simultaneously weakens the
hybridization between the $f$- and conduction electron states and encourages
complex magnetism. At concentrations that show magnetism, applied pressure
suppresses the magnetic ordering temperature and superconductivity is recovered
for samples with low disorder. These results reveal that the electronic
behavior in this system is controlled by a nontrivial combination of effects
from unit cell volume and electronic shell filling. Guided by this topography
we discuss prospects for uncovering a valence fluctuation quantum phase
transition in the broader family of Ce-based ThCr$_2$Si$_2$-type materials
through chemical substitution.
