  Robust galaxy cluster mass estimates are fundamental for constraining
cosmological parameters from counts. For this reason, it is essential to search
for tracers that, independent of the cluster's dynamical state, have a small
intrinsic scatter and can be easily inferred from observations. This work uses
a simulated data set to focus on photometric properties and explores different
optical mass proxies including richness, optical luminosity, and total stellar
mass. We have developed a probabilistic membership assignment that makes
minimal assumptions about the galaxy cluster properties, limited to a
characteristic radius, velocity dispersion, and spatial distribution. Applying
the estimator to over 919 galaxy clusters with $z_{phot}<$0.45 within a mass
range of $10^{12.8}$ to $10^{15}$ M$_\odot$, we obtain robust richness
estimates that deviate from the median true value (from simulations) by -0.01$
\pm $0.12. The scatter in the mass-observable relations is
$\sigma_{log_{10}(M|\mathcal{R})}=$0.181 $\pm$ 0.009 dex for richness,
$\sigma_{log_{10}(M|L_\lambda)}=$0.151 $\pm$ 0.007 dex for optical luminosity,
and $\sigma_{log_{10}(M|M_\lambda^*)}=$0.097 $\pm$ 0.005 dex for stellar mass.
We also discuss membership assignment, completeness and purity, and the
consequences of small centre and redshift offsets. We conclude that the
application of our method for photometric surveys delivers competitive cluster
mass proxies.
