  Fermi surface fluctuations and lattice instabilities in the 2D metallic
kagome superconductor CsV$_3$Sb$_5$ are elucidated via polarization-resolved
Raman spectroscopy. The presence of a weak electronic continuum in high-quality
samples marks the cross-over into the charge-density-wave (CDW) ordered phase,
while impurity-rich samples promote strong defect-induced electronic scattering
processes that affect the coherence of the CDW phase. CDW-induced phonon
anomalies appear below $T_{\mathrm{CDW}}$, with emergent $C2$ symmetry for one
of the CDW amplitude modes, alluding to nematicity. In conjunction with
symmetry-breaking lattice distortions, a kink-like hardening of the A$_{1g}$
phonon energy at $T_{\mathrm{CDW}}$ signifies a concerted interplay of
electronic correlations and electron-phonon coupling in the exotic CDW order.
