As good as strategic endorsements are, its probably not feasible given union’s structure in the us. I’m not a UAW member, so I may be misinformed on specifics, but these things are usually done as a vote at a union convention, by union members who are volunteering to be a voting delegate on top of their normal job.
Scheduling an emergency meeting to decide on endorsements pending a commitment from a candidate is not something you can reasonably do, especially when the voting membership are volunteers. Unions are democratic organizations, and can’t act quickly enough for strategic endorsements like you propose.
Democrats in the white house means better members on the national labor relations board. That alone is enough to justify an endorsement even without any legislative concessions.