  With the IRAM 30 meter telescope, we performed observations of the $J$ = 2-1
transitions of CS, C$^{33}$S, C$^{34}$S, C$^{36}$S, $^{13}$CS,
$^{13}$C$^{33}$S, and $^{13}$C$^{34}$S as well as the $J$ = 3-2 transitions of
C$^{33}$S, C$^{34}$S, C$^{36}$S, and $^{13}$CS toward a large sample of 110
HMSFRs. We measured the $^{12}$C/$^{13}$C, $^{32}$S/$^{34}$S,
$^{32}$S/$^{33}$S, $^{32}$S/$^{36}$S, $^{34}$S/$^{33}$S, $^{34}$S/$^{36}$S, and
$^{33}$S/$^{36}$S abundance ratios with rare isotopologs of CS, thus avoiding
significant saturation effects. With accurate distances obtained from parallax
data, we confirm previously identified $^{12}$C/$^{13}$C and $^{32}$S/$^{34}$S
gradients as a function of galactocentric distance (RGC). In the CMZ,
$^{12}$C/$^{13}$C ratios are higher than suggested by a linear fit to the disk
values as a function of RGC. While $^{32}$S/$^{34}$S ratios near the Galactic
center and in the inner disk are similar, this is not the case for
$^{12}$C/$^{13}$C, when comparing central values with those near RGC of 5 kpc.
As was already known, there is no $^{34}$S/$^{33}$S gradient but the average
ratio of 4.35~$\pm$~0.44 derived from the $J$ = 2-1 transition lines of
C$^{34}$S and C$^{33}$S is well below previously reported values. A comparison
between solar and local interstellar $^{32}$S/$^{34}$S and $^{34}$S/$^{33}$S
ratios suggests that the Solar System may have been formed from gas with a
particularly high $^{34}$S abundance. For the first time, we report positive
gradients of $^{32}$S/$^{33}$S, $^{34}$S/$^{36}$S, $^{33}$S/$^{36}$S, and
$^{32}{\rm S}/^{36}{\rm S}$ in our Galaxy. The predicted $^{12}$C/$^{13}$C
ratios from the latest GCE models are in good agreement with our results. While
$^{32}$S/$^{34}$S and $^{32}$S/$^{36}$S ratios show larger differences at
larger RGC, $^{32}$S/$^{33}$S ratios show an offset across the entire inner 12
kpc of the Milky Way.
