  We present a systematic survey for mass-transferring and recently-detached
cataclysmic variables (CVs) with evolved secondaries, which are progenitors of
extremely low-mass white dwarfs (ELM WDs), AM CVn systems, and detached
ultracompact binaries. We select targets below the main sequence in the Gaia
color-magnitude diagram with ZTF light curves showing large-amplitude
ellipsoidal variability and orbital period $P_{\rm orb} < 6$ hr. This yields 51
candidates brighter than G=18, of which we have obtained many-epoch spectra for
21. We confirm all 21 to be completely -- or nearly -- Roche lobe filling close
binaries. 13 show evidence of ongoing mass transfer, which has likely just
ceased in the other 8. Most of the secondaries are hotter than any previously
known CV donors, with temperatures $4700<T_{{\rm eff}}/{\rm K}<8000$.
Remarkably, all secondaries with $T_{\rm eff} \gtrsim 7000\,\rm K$ appear to be
detached, while all cooler secondaries are still mass-transferring. This
transition likely marks the temperature where magnetic braking becomes
inefficient due to loss of the donor's convective envelope. Most of the
proto-WD secondaries have masses near $0.15\,M_{\odot}$; their companions have
masses near $0.8\,M_{\odot}$. We infer a space density of $\sim 60\,\rm
kpc^{-3}$, roughly 80 times lower than that of normal CVs and three times lower
than that of ELM WDs. The implied Galactic birth rate, $\mathcal{R}\sim 60\,\rm
Myr^{-1}$, is half that of AM CVn binaries. Most systems are well-described by
MESA models for CVs in which mass transfer begins only as the donor leaves the
main sequence. All are predicted to reach minimum periods $5\lesssim P_{\rm
orb}/{\rm min}\lesssim30$ within a Hubble time, where they will become AM CVn
binaries or merge. This sample triples the known evolved CV population and
offers broad opportunities for improving understanding of the compact binary
population.
