Models predict that water-rich planets may be common around low-mass stars, but definitive evidence for the existence of water worlds has remained elusive. Recent JWST observations of sub-Neptune sized planets orbiting nearby M dwarfs found atmospheres enriched in CH4, CO2 and H2O, but the enrichment mechanism is debated. To answer this question, it's important to know if these planets represent the broader population of sub-Neptunes, and to quantify the range of possible sub-Neptune atmosphere types. A first step is to constrain the volatile atmospheric mass fractions of more small planets, which requires precise planetary masses and radii. For near-resonant multi-planet systems exhibiting transit timing variations (TTVs), we can measure these planetary bulk densities. TESS has detected many such systems and obtained an initial set of TTVs, which we use as the basis for extensive follow-up. We have combined dozens of ground-based transits with TESS data to characterize six multi-planet M dwarf systems. Our preliminary survey results reveal several new small volatile-rich worlds, including some that are top candidates for atmospheric characterization with JWST and some of the coldest known volatile-rich rocky planets.