for ballot stuffing to work and to not get caught, you have to wait until after the legitimate votes are all received, then you can forge fake ballots for anyone or everyone who didn't already vote.
similarly, no process is perfect and you need people in place to weed out and destroy duplicate ballots. it requires a lot of communication and shared data access and transportation to disperse the ballots.
You need them to appear to be from all voting precincts too. this complicates things for any conspiracy and makes catching them easier.
another challenge is the post office scans ballots it sent out and scans ballots it receives. they also photograph every piece of mail that goes through sorters and save the image with it's source post office and destination address recorded. (it's not a huge leap for infrared fingerprint scans as well, but that's pure speculation.)
ballot stuffing would be easily identified by any even marginal attempt at dashboarding the ballots outgoing and incoming by zip code. watch for the outliers.
the 2018 midterms probably were used to identify everyone with the intent and capacity to ballot stuff.
i suspect everyone planning this year's vote by mail ballot stuffing is already under surveillance and all the organizations have already been infiltrated by DHS and FBI. hopefully they all get bottled up the week before the election.
https://twitter.com/realDonaldTrump/status/1301100620741595136
POTUS: "Rigged Election?"