You can have an income- and wealth-dependent fine.

It could be as a % of your total income (bonus included) + wealth (including stocks, full ownership chain included).

1.5% of income, 5% of wealth in bank account, 10% of stock value. If any of these three exceeds 300x the median of the lowest 10%'s salary (let's say that that's 10k), then it's 80% for all three. That would hit the ultra-wealthy more and so it'd be fairer.

For a poor person earning less than €10,000 a year and having €1,000 in bank and no stocks, that'd be €200.

For someone earning €40,000 a year, having €80,000 in bank and €10,000 in stocks, that'd be €5,600.

For a billionnaire who has 1 billion, let's say €2,000,000 per year as "income", €100,000,000 in "bank account", and the rest of 1 billion in stocks. Then they'd be paying € 801.6 million.