Any Senate appointment in TX is good only for six months. A special election is required within six months.
Certainly, Greg Abbott and his Lieutenant Governor Dan Patrick can influence the process (and make that race into a successful spring training for dumping Cornyn).
If Trump wants to make his bones by reigning in what passes for GOP "leadership" in the two houses of Congress, that would be a good demonstration project for his ability to fundraise and to influence voters. Then set priorities for a rolling purge of the quisling senators of states like Tennessee, Mississippi, Louisiana, Virginia, North Carolina (from Helms and East it has degenerated to Tillis and Burr?), Arizona, Orrin Hatch, Lindsay Graham, Claire McCaskill, Mark Nancyboy Kirk, Tammy Baldwin, and soooo many others. An appointment here, a resignation there, a scandal, primaries, whatever port in the storm. Play for keeps. Clean 'em out!