The entire state of Oregon has done vote-by-mail for the past ~30 years and it seems to have gone fine for them for decades. Then again, their voting by mail wasn't being challenged by an actively hostile Federal government for much of that time.

I'm not American, but if I'm not mistaken, you don't have election day off like in other countries, right?

If so, how is that not clearly discriminatory for those who can't afford the time to go.

Then voting needs to be a national holiday with plenty of polling locations.

How much fraud is proven to be occurring due to mail-in ballots?

It seems to be nearly a third (short read): https://electionlab.mit.edu/research/voting-mail-and-absente...

How much fraud happens because of mail-in or early voting?

That's just fear mongering. Absentee voting by mail has been a thing for centuries. Attempting to limit it to people who are unable to vote in person doesn't do anything to make it more secure, and is a very weak control. It's not like they're going to check up on every single person who claims to have a specific reason they can't get to the polls. Beyond that, no-excuse absentee ballots have been a thing in many states for decades. Three states have been mailing absentee ballots to every voter automatically for a decade or more. Despite this long history, there is no evidence of widespread fraud or other problems resulting from it.

Perhaps every state should just have one very well-monitored polling place.