I’m in favor of calling the bluff, but leave it peaceful:

• Army bases remain US army bases and the national guard remains US. Texas must raise its own military without any US military equipment. Existing US servicemen must resign if they wish to join the Texas military, and will be treated as foreign army – they must leave US bases.
• Any companies headquartered in Texas will be considered foreign companies and subject to all relevant taxes and laws. Employees will be considered employed by foreign companies. Any subsidizes and other credits to these companies are forfeit.
• Employees of the state government will be considered foreign state agents.
• Trade deals must be negotiated with the US. Any US facilities providing goods and services, like water or energy, will now charge a fair market rate.
• Texas must renegotiate trade with other countries.
• For the first five years, Texas and the US must allow people to freely move out of or into Texas. If any Texas resident wishes to live as a US citizen, Texas must pay for their relocation.
• After that, or if they choose to renounce US citizenship, Texans are considered foreigners and will be treated as illegal immigrants if they enter the US without proper documentation.
• Texas universities lose all US accreditation. Current Students may transfer to a US university at no cost, and have their existing credits recognized. Texas will pay for any moves. New students from Texas will be considered foreign students and applicants for admissions and tuition at US universities.
• Any attack on US people or property or facilities will be seen as an act of war.
• Texas senators and house representatives are expelled from Congress.

They’ll be crying and begging to rejoin the US within a month. They’ll be a fourth world hell scape within a year if they don’t.