Low-mass X-ray binaries have long been theorized as potential sources of continuous gravitational-wave radiation, yet there is no observational evidence from recent LIGO/Virgo observing runs. Even for the theoretically 'loudest' source, Sco X-1, the upper limit on gravitational-wave strain has been pushed ever lower. Such searches require precise measurements of the source properties for sufficient sensitivity and computational feasibility. Collating over 20 yr of high-quality spectroscopic observations of the system, we present a precise and comprehensive ephemeris for Sco X-1 through radial velocity measurements, performing a full homogeneous re-analysis of all relevant data sets and correcting previous analyses. Our Bayesian approach accounts for observational systematics and maximizes not only precision, but also the fidelity of uncertainty estimates - crucial for informing principled continuous-wave searches. Our extensive data set and analysis also enables us to construct the highest signal-to-noise ratio, highest resolution phase-averaged spectrum of a low-mass X-ray binary to date. Doppler tomography reveals intriguing transient structures present in the accretion disc and flow driven by modulation of the accretion rate, necessitating further characterization of the system at high temporal and spectral resolution. Our ephemeris corrects and supersedes previous ephemerides, and provides a factor three reduction in the number of templates in the search space, facilitating precision searches for continuous gravitational-wave emission from Sco X-1 throughout the upcoming LIGO/Virgo/KAGRA O4 observing run and beyond.