  Symmergent gravity is an emergent gravity model with an $R+R^2$ curvature
sector and an extended particle sector having new particles beyond the known
ones. With constant scalar curvature, asymptotically flat black hole solutions
are known to have no sensitivity to the quadratic curvature term (coefficient
of $R^2$). With variable scalar curvature, however, asymptotically-flat
symmergent black hole solutions turn out to explicitly depend on the quadratic
curvature term. In the present work, we construct asymptotically-flat
symmergent black holes with variable scalar curvature and use its evaporation,
shadow, and deflection angle to constrain the symmergent gravity parameters.
Concerning their evaporation, we find that the new particles predicted by
symmergent gravity, even if they do not interact with the known particles, can
enhance the black hole evaporation rate. Concerning their shadow, we show that
statistically significant symmergent effects are reached at the $2\,\sigma$
level for the observational data of the Event Horizon Telescope (EHT) on the
Sagittarius A* supermassive black hole. Concerning their weak deflection angle,
we reveal discernible features for the boson-fermion number differences,
particularly at large impact parameters. These findings hold the potential to
serve as theoretical predictions for future observations and investigations on
black hole properties.
