You just need to find the right balance.
In my country we still rely on a paper process - which is very hard to tamper with but also has a bunch of problems - recently we needed to repeat local state elections because of such problems including shortages of ballot papers, unusually long queues to vote, ballots being delivered to the wrong locations, and in some cases voters being turned away. Many of these problems go away with digital voting - but then you have other issues.
I don’t think a perfect solution can exist
Not even a remote access to a demo ballot machine.
Our congress has determined that a paper trail must be implemented but the judges aren't having it.
https://news.ycombinator.com/item?id=36543423
Source code is a red herring too. If there's no reproducible builds that cryptographically establish source-executable correspondence, showing us the source code would prove nothing and only serve to embarrass us. Supply chain attacks could compromise it without any source code vulnerability. What they need to do is publish the signed executables that ran on election day as well as every executable used to construct those artifacts. Our AIs will make short work of the binaries.
In any case, the prevailing theory is that the mesários are partidarian and simply impersonate people who didn't show up to vote in order to boost their candidate's numbers. Very low tech, can be fixed with surveillance: have people from all parties everywhere observing everything. It boggles my mind that this isn't being done already.