Should I pay off a 40-year 1.5% mortgage early?

I live in Europe and I took advantage of a government program for first time home buyers. I took a 100k loan with 10k down, to buy a house. The loan term is 40 years.

For the first 6 years, government is subsidizing my mortgage, so I basically have a 1.5% rate, monthly payment of 330 USD.

After 6 years, it converts to variable interest rate, which would be ~4.2% currently, and monthly payment of ~500 USD, but no one can tell for sure what the situation will be in 6 years.

Our household take home pay is 4200 USD a month, so the payments aren't a problem, but what worries me is the length of the loan and the vatiable interest rate. Even if the loan remains for the full 40 years, if I lowered the payment to something like 100-150 USD, I wouldn't mind the length of the loan that much.

What would be the best play here? Invest in stock market while the interest rate is 1.5%, then after 6 years start paying it off early? Save some money for the 6 years in a money market account and then throw a lump sum towards the principal? Or just pay off 2-3k a year starting immediately, so when subsidies expire and variable rate kicks in, the payment is not 500, but more like 350 USD?