Selling stocks in foreign broker vs taxable account

So for my understanding: as a permanent tax resident, when I sell stocks/etfs that are in my foreign broker, then I have to pay not only ~20% tax, but it is also considered income so it will increase my health insurance contributions, and as employee also my pension contributions. But if I sell stocks/etfs from my taxable japanese broker account then I have to only pay the ~20% tax (which is already deducted) but do not have to do a tax declaration and it has no impact on health/pension payments?

by klimaheizung