As far as I can tell, the majority of the cost of the development of the technology to take millions of tons to Mars is being funded by a private company.
That company's revenues, insofar as they result from what were once public funds, are primarily a result of projects related solely to the Earth (and perhaps one planned mission to the moon).
NASA is not paying for SpaceX to go to Mars. NASA is paying SpaceX to take stuff to Earth orbit.
That's always been a stated goal of SpaceX to do exactly that. That's why they funded on their own dime the development of a massive reusable vehicle that has literally no reason to be that large unless your explicit plan is to design it for interplanetary cargo.
> Almost all of SpaceX's income, absent Starlink subscription fees, traces to tax dollars.
Not true. As stated explicitly by SpaceX in several years ago, the revenue split of SpaceX was 1/3 NASA, 1/3 other government and 1/3 commercial. But in recent years the commercial fraction has dramatically increased (simply as evidenced by the tremendous numbers of missions happening from non-government customers).
Uhm, they do a lot of purely commercial space launches (comsats, LEO missions, commercial dragon flights) even including for some of their direct competitors to Starlink (OneWeb)!
Establishing a self-sufficient city on Mars does benefit us, even if it is more difficult.