I suspect without ISRU production of bulk orbit sheet metal, the most feasible solution is to repurpose rockets in their whole.

Building a station this large is gonna be costly even within the cargo hold of starship. But six of them, gutted of insides as welded end to end could provide the vast majority of the bulk mass.

This assume rather sophisticated orbital welding and object manipulation; but its feasible we could do it with robots.

This was considered with the orange Space Shuttle fuel tanks; they went almost all the way to orbit anyways.

aren't rockets like the starship almost the opposite of what you want in a space station? They want to minimize the integrity of the rocket as much as possible (without blowing up) to reduce the mass while for a station you want robustness (for pressure & impacts).