  Stellar evolution models of massive stars are very sensitive to the adopted
mass-loss scheme. The magnitude and evolution of mass-loss rates significantly
affect the main sequence evolution, and the properties of post-main sequence
objects, including their rotational velocities. Driven by potential
discrepancies between theoretically predicted and observationally derived
mass-loss rates in the OB star range, we particularly aim to investigate the
response to mass-loss rates that are lower than currently adopted, in parallel
with the mass-loss behavior at the "first" bi-stability jump. We perform 1D
hydrodynamical model calculations of single $20 - 60 \, M_{\odot}$ Galactic ($Z
= 0.014$) stars where the effects of stellar winds are already significant
during the main sequence phase. We develop an experimental wind routine to
examine the behavior and response of the models under the influence of
different mass-loss rates. This observationally guided, simple and flexible
wind routine is not a new mass-loss description but a useful tool based on the
Wind-momentum Luminosity Relation and other scaling relations, and provides a
meaningful base for various tests and comparisons. The main result of this
study indicates a dichotomy when accounting for currently debated problems
regarding mass-loss rates of hot massive stars. In a fully diffusive approach,
and for commonly adopted initial rotational velocities, lower mass-loss rates
than theoretically predicted require to invoke an additional source of angular
momentum loss (either due to bi-stability braking, or yet unidentified) to
brake down surface rotational velocities. [...]
