People around here definitely expect you to “do the work” on this yourself, which is understandable. However, I do want to set some expectations for you as someone who has learned a lot on this over the last month and half or so and very seriously considered the A6 e-tron – I don’t think you have any prayer of getting to 0 down, payment under $500 on this particular car with that mileage requirement. You could maybe get low $500’s if you can find a dealer to give you an amazing discount on MSRP on the base trim.
Someone in Texas (which I think has state level incentives? not sure on that) got a Premium Plus RWD for $328/mo w/ $2500 down but that was at 7500 miles per year. I don’t know how replicable that is for CA.