It's likely that the transmission lines are less of a bottle neck than the frequency conversion. Power lines are pretty darned efficient, and given the cold weather, theyd be more efficient than usual too
Electrical engineers wouldn't build a power line to Texas with a capacity of 5000 MW and then put intertie hardware such as phase shifters or ac-dc-ac converter with capacity of 1000 MW on the end of it. So it is likely that the transmission capacity and intertie capacity are exactly the same!
They make long distance DC innerconnects, while expensive they are resistant to particular types of problems and can connect to multiple different grids at the same time.
The Chinese have an amazingly ambitious plan to shift power from one part of the country to another with UHV DC. It has its own challenges, but it is definitely the way to shift massive amounts of power without grid interconnect (and the resulting frequency stability issues). Much lower loss, even with DC-AC converters.