Oregon, Washington, Colorado, and Utah have voted by mail and sent a ballot to every registered voter for years. No signs so far that this has been abused to facilitate the sort of coordinated large scale fraud that you’re concerned about. Though there’s some evidence that it leads to closer matches between the votes of married partners compared to in-person voting, which is very concerning for for similar reasons! But spousal vote coercion is not new and by most definitions is not a form of voter fraud. I dream that someday we will be able to introduce a voting system that can ensure ballot secrecy even in these scenarios, but sadly the technology isn’t there yet. (Side note - this is why taking a picture of your ballot in a voting booth is illegal. It prevents being able to prove how you voted.)

Ballots are delivered to the legal address of each registered voter. A state election board is going to notice (and maybe call the fire marshal haha) if 1,000 people all suddenly announced that they are moving into the same house. The premise that neither the state nor the people having their legal address surreptitiously changed would notice isn’t a realistic cause for concern.