In my experience, most of the complaints relate to ID verification, and auditability. And a lot of those complaints involve a misunderstanding of how the current system actually works.

I think if you centralize registration at the DMV, which is already in the business of verifying identity, that would ensure that the correct voter keys are issued to the correct, verified voter. They could issue a voter ID card with a photo and appropriate voter information, as well as a QR code to be scanned at in-person voting centers, and something like a seed phase that could be entered on mail ballots confirm the eligibility and identity of the voter.

Mail-in and drop-box ballots are inherently paper ballots, and a paper receipt could be generated at in-person voting centers to both assure the voter that their correct selections were submitted, and as a physical backup that could be used along with the mail ballots to perform a hand audit if necessary.