24M buying first ap

In the next 2 months I will sign on a new apartment at around 110k with the bank( I will need to buy furniture for it). i already payed 6k as reservation fee. My finances look like this: By august I will have 50k in my bank account . I am making around 5.5k euro net in the “bad” months, but it may be months when I do also 8,5k. It can t go under 5,5k just if I lose one of the jobs ( I m employed + freelance contractor). Would you invest something in ETFs etc or try to pay the mortgage ASAP? My m