> You could consider spoiling your ballot instead?

No. Either vote for one of the third party candidates (doesn't much matter which one), or vote write-in for a fictional character or "none of the above" or something.

Ballots are generally counted as long as they're legible. Voters aren't required to pick a candidate in every race, they can leave some blank. And in most states there were third party candidates listed as well.

Those rules are established by the states, but in general, they are not. A spoiled ballot is equivalent to no vote.

The states do track undercount votes, But they don't mean anything in terms of the law that decides representation.