Hi Zafra - I share a similar "cold" problem at 23 latitude in Baja California with summer highs in the 90s and winter lows generally mid- 50s. Humidity generally ranges from 25 (winter) to 65 (summer). This is a new climate to me, so I'm keen to find others to share experiences.
The citrus I have and my interest in growing them was all inherited when I bought the house 4 years ago so I'm no expert. I don't even know what varieties I have. There are Valencia and Mandarin oranges, lemons, Mexican limes and yellow grapefruit which all seem to thrive here and produce tasty, juicy fruit (again, I'm no expert on quality).
The grapefruit tree produces the only grapefruit that I've ever enjoyed eating. My wife is a grapefruit fan and says they are the best she has ever tasted. They are less sour than other grapefruit and are so sweet that they never need sugar. They are also very juicy. Maybe it is the variety but I haven't identified it. I'll add images if it can help identify it and can add more specific photographs on request to help identify it.
My wife also pointed out that pink grapefruit are usually the sweetest, so you could try growing those.
Not sure if it has anything to do with fruit quality but the tree was severely traumatised in the past. It was in very poor shape with loads of dead branches when I inherited it 4 years ago. The garden had been neglected for about 10 years and the grapefruit was overgrown by another tree which almost enveloped it.
Close-up of leaves and fruit (about 5" diameter)
The tree on Feb 16. We have been picking fruit for a couple months so it is looking a bit depleted.