It's too cold when the pup looks unhappy. If she is happy, it's not too cold.
When you are too cold, though, it is probably time to come inside.
Generally a dog does not need boots for an outing of reasonable length on walking paths. The usual reason that people use them is to protect pads from road salt which stings the poor pup. Or to protect from continually forming iceballs that make the walk difficult for the pup. If your pup is stopping and lifting her paw that's usually a sign of discomfort.
Dogs who do certain sports, dogs who have to traverse certain kinds of terrain, dogs who tend to gather iceballs quickly and continually and so cannot walk, dogs who are handicapped, might wear shoes even when there is no salt on the path.
Many of mine hated shoes, and honestly trying to keep them on was more of a pain than it was worth. So for them my rule was that we stopped when I could no longer keep up with the ice balls.
However, I had a couple of handicapped dogs who needed shoes. Melon, the collie of my user name, had an extensive collection. In his case, shoes allowed him to enjoy the snow, sun, and fresh air, and contributed to his quality of life.
But be warned: You have to train your pup to wear shoes. (The initial reaction is often worthy of the 'ministry of silly walks'.) Some learn to accept them, others never do.
But before you go down the shoe route, perhaps try paw wax.
Musher's Secret if you can find it in Switzerland, or Fyra Ess salve are two I like, but there are other brands available at most pet stores. Put the wax on just as you are going out, your pup standing on a towel, as it can stain some floor types. And bring a small tub with you, as you might need to re-apply it on the walk.
ETA:
As for a fleece: Again, depends on the dog. Breeds with very little body fat such as Greyhounds have trouble in the cold and so often need a fleece when it gets below freezing. My collies and shelties, haystacks on legs, with their long insulating double coats, laugh when the thermometer turns blue. Coats completely unnecessary, and more of a hindrance, for them. They were made for the outdoors. My poodle felt the cold much more. His coat, while fluffy, was single, and not insulating. He was also rail thin. So he was happier in a coat.
If you do go with a coat, make sure it is waterproof! A non-water proof fleece or knit coat can quickly become a wet blanket, making the dog even more uncomfortable. Also make sure the dog can easily and cleanly piddle in the coat - for obvious reasons.
Your little one is eight months already?
Why have we not seen more photos? Hint, hint.