Why is my sourdough too sweet?
Lack of sour indicates more yeast activity over the souring lactic acid bacteria (LAB), and using a firm starter from that link favors yeast and not LAB. For more sour, increase the water content. There's a bunch of other dials you can fiddle with, Ken Forkish in "Flour Water Salt Yeast" indicates that a "warmer levain cultures encourage lactic acid production", your 16 °C favors acetic acid production. Keep the temperature around 26C to 32C to favor LAB. (Perhaps use an insulated box to help avoid the nighttime temperature swings?) Another point would be what temperature water are you hitting it with; tap water during winter could be quite cold (I use room temperature water that has stood to evaporate off the municipal chlorine). Other things to experiment with might be adding small amounts of salt (though this will slow down the ferment) or changing the flour or mix of flours used; whole grain vs. not, rye vs. wheat, etc.

A notebook may help; track ambient temperatures (min/max), temperature of the water used, amount, type of flour, plus any notes on what the starter looks and smells like.