There is a simple solution here: everyone gets a serial number with their ballot, the list of serial numbers for each candidate is published, and you can verify

There is a simple solution here: everyone gets a serial number with their ballot, the list of serial numbers for each candidate is published, and you can verify that your vote was counted in the right spot. Yes, this is susceptible to coerced voting where your family member can force you to vote a certain way, but guess what, the entire vote-by-mail system that we've gone to is already susceptible to that, so as far as I'm concerned the ship is sailed.