I invested the same amount in an index fund which can earn an average of 10% annually I could have €361,000 saved after 10 years
By my reckoning, €20k per year invested monthly at an annual rate of 10% only yields €341k. Presumably you would only pay 41% on the profit of €141k.
Guys may I suggest that you are out of your depth here, and at least one of you cannot count.
Talk about "an index fund which can earn an average of 10%" is not serious. If you don't understand that, keep away from investing.
A guaranteed and after tax return of 3.1% from paying off a mortgage is a serious possibility.
Maximising the tax shelter available for pension contributions is also worth consideration.