It would be worth looking into which states have wording in their Constitutions that include “shall certify” which would make the stalling tactic a crime.

Also, anyone who voted in a state whose electors were not sent on time would have a valid civil rights case against that state’s officials and they would all have standing because they were personally harmed. The number of potential lawsuits would be overwhelming and perhaps ruinous to anyone found guilty. Not sure that fear of that would be enough to stop them, but also they have to worry about losing their next election (though if it worked they might hope to be rewarded by the winning administration).