Syngas fermentation and chain elongation are key anaerobic biotechnologies for waste carbon upcycling. Their integration as a mixotrophic process enables simultaneous conversion of gaseous and wet waste substrates into medium-chain carboxylic acids and alcohols with high yields and no CO2 emissions. However, in practice, open culture-based processes suffer from low product yields, poor electron selectivity, and a narrow product range. Here, we explore synthetic consortia as a platform to advance one-pot mixotrophic waste conversion to medium-chain oleochemicals. We propose strategies for building synthetic consortia through a top-down, bottom-up approach, leveraging automation and high-throughput microbiology to accelerate bioprocess development. These advances could improve yields, expand waste feedstocks, and produce new chemicals, accelerating carbon-efficient waste upcycling toward industrial adoption while driving the circular economy.