The Texas grid appears to be something less 100% freestanding. There appear to be 1 connection via Oklahoma and 2 connections to Louisiana that connect to the national grid. There's one connection into New Mexico that's probably an extension of the Permian Basin but didn't appear connected to the national grid. The region represented by El Paso appears to wholly on the national grid and not to the Texas Grid. The far west counties of the panhandle may also be totally on the national grid via New Mexico and not the Texas grid.
Bottom line though, connection wise it appears Texas could be fully independent with a few switch flips. Caveat though is that I have no idea if Texas has sufficient generation supply to fulfill demand. I bet they do but am not positive. I know that in the 1980s we had a 1500MW cogen power plant trip offline when it caught on fire. The Texas grid absorbed the loss without skipping a beat.
Since Texas would keep fracking going full speed, and also having large reserves of coal, I think they would not only be self sufficient in power, but able to make a lot of money selling to the USA which will be crippled by the UN carbon crusade.