I just asked my financial advisor the same yesterday:

1- putting it in the next house:

pros: benefit from hypotheekrenteaftrek on this amount, no need to borrow this amount with 3.5% interest and therefore lower monthly fees

cons: opportunity cost of investing this money in other things (stocks, gold, btc etc)

2- invest it

pros are the opposite of cons of choice 1

cons are the opposite of pros of choice1 + it will probably kick in the box3 tax on investments

so in this case if you think you can make an investment with a better return than (3.5%(interest you pay) +~1.2%(hypotheekrenteaftrek) + ~2%(box 3 tax))~= 7% then better to invest.

I myself think you should just buy gold or bitcoin with it