Why does voting need to be automated at all? Elections have run fine for hundreds of years with manual counts.
I mean, run the elections on Saturday instead. Then you'd have a whole weekend to count without work getting in the way
Of course the US is large, but I don't see how that's a limiting factor, being the process naturally very parallel.
If you can verify your own vote you can use the same method to verify your vote to someone else, opening up the voter to the pressures that motivated voter privacy. Like payments for voting right or threats for voting wrong. So in some measure individual vote verification would trade transparency for vote coercion and selling.
Which is why absentee voting and ballot harvesting are so contentious.
The reason every functioning democracy uses a secret ballot is that coercion and bribery were trivial wherever the ballot wasn't secret. Bribery might sound uneconomical for a national election, but there will be plenty of local elections using the same machines where it could easily make the difference.