To put a sharper point on your point:
Title 18 of the United States Code (U.S.C.) under Section 611 makes it a crime for noncitizens to vote in any election for President, Vice President, presidential elector (and certain other federal offices).
The time to enforce that law is at the Special Session following each presidential election. The Congress is allowed to reject certificates from states who submit invalid certifications; certainly a violation of a federal statutes would qualify.
It is not a voter-by-voter analysis it could be argued a state's election was conducted in manner that clearly allowed multiple violations of the law.
Our representatives in Congress may hold our last remaining opportunity.
Note my post 22. If E Check were required but not run it would be clear there were multiple violations in the time between an election and acceptance of electors. Though it should be used prior to elections, 365 days a year.