As a Colorado resident, voting "by mail" as in, receiving a ballot in the mail and then dropping it at a ballot drop box, has been the only way I've ever voted.
It's a great, low-friction system. I've never dealt with crowds nor had any trouble with ballot box access.
The only issue I can imagine coming up is people without a mailing address, but we do have traditional polling places (that are much less crowded) for that reason.
We need these fascists out and we need this system adopted nation-wide to give more people their voices.
Colorado also has a system called BallotTrax iirc that lets us know if our ballot has been counted. Other places should do that too. It's great.
My only other criticism of our system is that we don't have ranked choice voting, but we'll get there.
The ballot measure for it a couple years ago failed because honestly it wasn't worded in a way that people understood what they were voting for