My state has had mail-in voting for many years. The local news had a few interesting statistics. Less than 10% of people voted at polling stations and two thirds of the ballots were returned not by the post office, but by drop-off boxes located around the cities. It has never caused any problems and is far more convenient. I suspect with all the haggling delays it would be hard to gear up systems for mail ins by Nov., but the drop boxes partially solve the concerns about overwhelming the post office for timely returns.
I have vague recollections of the last time I voted at a polling station. There was an early snow storm that messed up the streets, plus a rush to get to the polling station after work. I don't understand why it is not more common if the issue is to have as many people vote as possible.