This I believe is true and one of the reasons why electronic voting machines are not the issues, fraudulent ballots are a much bigger issue.
Once a ballot is turned in, regardless of its validity, it will get counted, there will be no judge anywhere that will do anything about it, if the illegal ballots get counted, does it really matter whether it’s by hand or electronic machine, the results will not change.
You have correctly identified the issue ... the ballots themselves and more specifically mail-in balloting. As you say once the fraudulent ballots are blended with the legal ballots and counted, it is difficult to change things.
Mail-in ballots must be restricted to the verifiably disabled and out of country USA citizens - only.