Have a really good lawyer who anticipates this sort of dispute. I would bet that the contract was written in a kind of "tricky" way that didn't make it clear exactly when the royalties applied.

I think the artist (unless they're like Radiohead or Taylor Swift or something) is always going to be at a massive disadvantage in any negotiation with a label (massive corporation).

This is just the nature of capitalism. The more capital you have, the more leverage you have.

The people who actually create the product are always at a disadvantage compared to those who own the means of that production.

You can try pooling resources together to negotiate collectively (unions), but without government intervention to put a thumb on the scale to try to even the playing field for unions vs corporations, even that would usually end poorly for the people who produce the actual product (see: union busting, the Pinkertons, etc.). Historically, they may actually even straight up just kill you for trying.

Capital also equals political power (even moreso since Citizens United), and corporations use that power to do everything they can to prevent that balance from tipping any direction but theirs.

It's just business, unfortunately. You can engineer a solid contract, but then the side with the leverage will look at it and say no. And that side is probably not yours; otherwise, you probably wouldn't be selling your business.