The Netherlands manages to do exactly that.
> There's no way to (fairly) tax capital gains
The fair part is that it's taxed - otherwise, it's unfair on people who are unable to exploit this tax avoidance.
We are talking about people with meaningful wealth. They can afford to be taxed without waiting for it to be perfectly 'fair'. Letting them pay $0 until its fair doesn't make sense.
It makes more sense to pay the expected amount, at least until you have the receipts.
Indeed, that's how my own labour income is already taxed. I first pay income tax for 2021 based on my income in 2020 - whenever it turns out to be too much, I can declare it later and will get a refund. But, I can't pay $0.