  Analyses of upcoming galaxy surveys will require careful modelling of
relevant observables such as the power spectrum of galaxy counts in harmonic
space $C_\ell(z,z')$. We investigate the impact of disregarding relevant
relativistic effects by considering a model of dark energy including constant
sound speed $c_{\rm eff}^2$, constant equation of state $w$, and anisotropic
stress sourced by matter perturbations $\pi$. Cosmological constraints were
computed using cosmic microwave background anisotropies, baryon acoustic
oscillations, supernovae type Ia, and redshift space distortions. Our results
are consistent with $w=-1$, $c_{\rm eff}^2=1$, and $\pi=0$. Then, a forecast
for the performance of an Euclid-like galaxy survey was carried out also adding
information from other probes. Here we show that, regardless of the galaxy
survey configuration, neglecting the effect of lensing convergence will lead to
substantial shifts in the galaxy bias $b_0$ and the neutrino mass $\sum m_\nu$.
Shifts in the dark energy sound speed and anisotropic stress also appear, but
they depend on the survey configuration and hence lack robustness. While
neglecting lensing convergence also leads to a Hubble constant $H_0$ moving
downwards, the significance of the shift is not big enough to play a relevant
part in the current $H_0$ tension.
