When you are voting for a dozen candidates and a dozen ballot measures, it takes time to read through all of them and make sure you are marking the correct boxes, even when you know how you will vote in advance.
Here's an example from this year.
https://www.kingcounty.gov/~/media/depts/elections/how-to-vo...
Send them from an arbitrary public mailbox at a busy place, like a train station or a mall.
This should make tracking impractical, unless you expect "them" to scan for fingerprints.
It gets validated, then passed along for counting.
Second, if you want, you may drop your ballot into any drop box, or hand deliver it to elections, who will validate it and put it into the counting queue.
Uh, you're only supposed to vote once.
Now the ballots show up, you can fill them out for all family members, even if recently deceased, and vote for everybody. That spouse doesn't need to vote. The high-school senior is too busy to do their own voting. The truly ambitious can even grab ballots out of mailboxes, adding them to their pile.