Other than those who are out of state on Election Day or otherwise cannot physically get to the polls, end all mail-in voting. If cannot get off your butt to go to the polls and vote then tough; by your own actions you have squandered your right to vote.
How about this then: In Michigan (my home state) we have a voter ID law. You must show either a Driver's License or State issued ID in order to vote in person.
How about we require that the voter must write their Driver's License number or State ID number on the outer envelope of their mail-in ballot directly above the signature line. If the number doesn't match voter rolls, or is missing, automatic invalidated ballot.
That would at least make it much harder to manufacture mail-in ballots en masse.
The only way to ensure fair elections is to hold a one day, in person voting by properly qualified registered voters. The only exceptions would be narrowly defined absentee voters such as military personnel. A sufficient number of polling locations would be provided to handle voting turnout. No more early voting or mail-in ballots. It ain’t rocket science.