Agreed: this is the key question. One effort I follow in this direction is Terraform Industries[0] who are building exactly this type of system.

Their approach is PV + DCC (Direct Carbon Capture) and then simple carbohydrate synthesis, with the goal of establishing standalone autonomous systems that can generate valuable resources on their own in remote areas with ample sunlight.

They have a great blog where they go through their motivation for the approach from first principles [1].

[0]: https://terraformindustries.com/ [1]: https://terraformindustries.wordpress.com/home/