Mail in voting is still secret ballot - you have an internal "secrecy sleeve" that is in turn mailed in an outer envelope. The outer envelope signature is compared to the signature on file, and then the internal envelope is removed but is not opened in the same location.
That doesn't stop a determined group of canvassers from going door to door and pressuring people to fill out their ballots on the spot while they wait on their doorstep. It's also a perfect time to offer a bribe or incentive for a speedy completion.
There's no way that any form of remote ballot is going to be as secure as doing it in person. It's just not possible because you will never know the provenance. Like most things that involve the legal system, it's not the law abiding that we're worried about.
I actually wonder how viable it would be to devise a targeted ballot spoiling attack.
Picture this:
Have a canvasser or harvester pick up mail in ballots. They take it to a safe house and open the envelopes. If it’s a vote for the candidate they don’t like, put marks on it so it doesn’t get counted. Place it in a new return envelope.
You’ve just cancelled that person’s vote. If someone checked that their vote was counted they wouldn’t be able to see that it was spoilt.
You could potentially just throw away the ballot too, I doubt most people check that they were actually received. But it does seem slightly more likely to raise eyebrows. I’m sure you could chalk it up to just getting lost at the post office.
You can't just return your ballot in any envelope, you'd have to have access to excess return envelopes which is a tall task.
Also, I'm not sure what you mean by adding marks invalidating the ballot. If you fill out a ballot incorrectly, the poll workers will notify you that there are issues with your ballot.
In Colorado, the return envelope has a unique barcode and has to be signed on the outside. Not fullproof, since they could print a new envelope with the same code and forge the signature, but not as easy as just having a stack of return envelopes to use.
If there were canvassers knocking on doors and offering bribes for ballots you would hear about it here in Washington state. People here take their voting rights seriously.
https://www.sos.wa.gov/_assets/elections/wa_vbm.pdf
https://www.cnn.com/2020/09/11/politics/barr-secret-vote-fac...