And the most effective way to get copyright to expire on bigger holders is to set a fixed time limit.

I dislike the idea of having "most" copyright expire quickly while the rest doesn't, because longer copyright is going to disproportionately be on big works owned by big holders. I don't think you should have that as a goal at all.

So I think you should just go with a fixed time limit. Like the original 28 years.

And a mechanism to avoid abandonment is a good idea, but make it fair to everyone.

... which were expensive to create, generally. Remember, copyright is not intended as a "reward" or a tool for social redistribution, it is intended to incentivize creating new works.

An argument can be made that allowing the $100m movie production to pay for longer copyrights than the $1,000 amateur shoot gives the creators an incentive to run bigger budget risks, because they can monetize it better.

(I don't agree and I do think 28 years of blanket protection should be enough, but the argument is valid.)

And big projects with big advertising budgets probably recoup their money faster than small projects.