The nucleus of any form of democracy is this - "One person-one vote".
A procedure has been put in place in every state in order to vote. Essentially, in order to qualify, one must follow these 4 basic steps: 1) You must register to vote first; 2) You must be a US citizen 3) You must be 18 years old or older; and 4) You must vote in the state and precinct in which you reside.
In order for any vote to be fair, one must prove they have met those 4 qualifications. An ID is shown and the name on the ID is checked, checked off, and the person given a ballot. Easy as pie and simple as that.
Anything that differs from that puts the election at risk for fraud. The 2020 election, with its mail-in voting rather than in-person, put the results of the election at risk. It wouldn't matter if Trump won, the case for fraud could still be made.
In most elections, mail-in ballot are challenge votes anyway. They rarely come into play unless the election is really close. That's why in-person voting MUST be the only form of voting - because it does the BEST job of determining a fair election. The states, for the most part, have made voting much easier, i.e. early voting. No longer does one have only one day to vote; often standing in line for hours in who knows what type of weather or conditions. Most of the problems have been fixed.
In the end, these other arguments are frivolous. Who won in 2020? Doesn't matter now. Ballot harvesting? Don't allow mail-in voting or treat each mail-in ballot as a challenge vote. Only vote on one day? Give people multiple days to vote - it just has to be done in-person.