The other 30,000 square miles -- 3/4th of it -- is still available for farming. And it isn't like solar panels can only be placed on existing farmland. I've seen a lot of rooftop and backyard solar installed around where I live.
If there is a global supply chain collapse, then the price of fossil fuels will likely go through the roof, placing a greater demand for substitutes like ethanol, which will raise the price of food and feed crops as farmers switch to produce the more profitable fuel crops. Or if the government orders the farmers to only produce corn for livestock and people, then there's no effective fuel substitute.
What keeps the land fertile is not the corn monoculture of places like Iowa, but rather a massive amount of fertilizer. As we know from the US war with Iran, fertilizer availability is subject to a global supply chain collapse because the current production methods require a lot of fossil fuel for power.
If you want to reduce your dependency on the global fertilizer supply chain, you'll need a lot of cheap local power to make things like anhydrous ammonia locally.
The experience with agrivoltaics shows that solar power and agriculture can coexist profitably on the same land. At the very least it's not simply a one-or-the-other option, or as you characterized it, "is gone".
If you really want to keeps millions of acres land fertile to handle a possible future economic crisis, you should want severe limits on building new urban and low-density residential development on farmland. If you turn farmland into a suburb, that farmland is far more "gone", for agricultural purposes, than setting up solar panels.
The paper at https://farmlandinfo.org/wp-content/uploads/sites/2/2022/06/... estimates well over 10 million acres of agricultural land lost that way by 2040, which at 15,000+ square miles is far more than the quoted 10,000 square miles needed to get rid of most oil use.
So it seems to me that for your goal of making the US less affected by a global supply chain collapse, you should want to have a lot more PV farms.