[photochemsyn]

Staggered extraction cycles, where each round leads to a 10X increase in concentration, is one option. If you look up how D2O (heavy water, where the hydrogens are replaced by deuterium) is produced, the first step is often done that way. The initial stage is the concentration of HDO from H2O, which is naturally found at about 1 part in 3200. 400 ppm CO2 is 1 part in 2500, so it's comparable.

Proof-of-process for Starship fueling would be generating a stream of pure CO2 from air at a rate sufficient to feed into a methane production facility (which would require a similar stream of hydrogen from water processing) to produce around 500 tons of liquid methane in a reasonable amount of time. Possible?