there are lots of S&P500 companies in TX, but they gotta factor these in.
IF TX goes independent and not a part of US, doing business with US will be international trade. Some minor tariffs and quotas will be imposed.
No federal funding of any sort.

TX then has has impose same thing on goods that comes into TX.
Walmart, Safeway, Kroger, automobiles, and any necessities to luxury item will be an import, not a delivery.

IMHO, Odds are very slim at this time. Who knows near to distant future....