  We present comprehensive orbital analyses and dynamical masses for the
substellar companions Gl~229~B, Gl~758~B, HD~13724~B, HD~19467~B, HD~33632~Ab,
and HD~72946~B. Our dynamical fits incorporate radial velocities, relative
astrometry, and most importantly calibrated Hipparcos-Gaia EDR3 accelerations.
For HD~33632~A and HD~72946 we perform three-body fits that account for their
outer stellar companions. We present new relative astrometry of Gl~229~B with
Keck/NIRC2, extending its observed baseline to 25 years. We obtain a $<$1\%
mass measurement of $71.4 \pm 0.6\,M_{\rm Jup}$ for the first T dwarf Gl~229~B
and a 1.2\% mass measurement of its host star ($0.579 \pm 0.007\,M_{\odot}$)
that agrees with the high-mass-end of the M dwarf mass-luminosity relation. We
perform a homogeneous analysis of the host stars' ages and use them, along with
the companions' measured masses and luminosities, to test substellar
evolutionary models. Gl~229~B is the most discrepant, as models predict that an
object this massive cannot cool to such a low luminosity within a Hubble time,
implying that it may be an unresolved binary. The other companions are
generally consistent with models, except for HD~13724~B that has a host-star
activity age 3.8$\sigma$ older than its substellar cooling age. Examining our
results in context with other mass-age-luminosity benchmarks, we find no trend
with spectral type but instead note that younger or lower-mass brown dwarfs are
over-luminous compared to models, while older or higher-mass brown dwarfs are
under-luminous. The presented mass measurements for some companions are so
precise that the stellar host ages, not the masses, limit the analysis.
