You didn't specify the term (length) of the mortgage or the interest. This is important because it effects amortization rates. For example, if you take out that ammount for 30 years at 6% (common in the U.S. right now) you will make over $215k in payments (excluding taxes and insurance), but only have $50k in equity. If you sell your house, you end up with the same $50k as the bitcoin holder. Whether you come out ahead on the deal will depend on the rent paid by the hodlr, as well as local taxes and insurance.

The real error here is assuming Bitcoin will only 5x in 10 years. Hitting $250k by the end of next year is my base case for BItcoin. There are a number of positive headwinds that could push it much higher. In 10 years, I will be verry supprised if it's less than $1,000,000. Of course, I also be supprised if your house was still worth $300k.

(I did everythin in dollars becausse there is no Euro symbol on my keyboard)

Here's a mortgage calculator you can play with: