One downside of any sort of copyright registration requirement (probably especially one requiring ongoing renewals) is that it benefits corporations at the expense of individual creatives. Disney's lawyers are not going to forget to renew and the revenues involved mean that any registration is likely trivial. Not so for an individual author or photographer.

In fact, groups representing authors and photographers have opposed orphan works legislation in the past.

The US used to require affirmative action to gain copyright protection but this was phased out to be consistent with most other countries.

What constitutes 'work'. On one extreme, a studio spending millions making a movie is a single piece of work. On the other extreme, a photographer might take 100 photos in a day that he/she offers for sales. The number won't work for the photographer.

Not OP, but I would imagine that the USG isn’t in a position to efficiently and effectively accept lots of small payments. Working from that premise, you’d probably optimize for taking advance payment for 8 years with some way to retroactively pay if your work is suddenly worth more than $20.