Slice cucumbers into thin rounds (about 1/8 inch thick). Place in a colander, sprinkle with 1/2 tsp salt, and let drain for 15 minutes. Pat dry with paper towels.
In a large bowl, beat softened cream cheese with sour cream until smooth. Add minced garlic, lemon juice, remaining salt, and pepper; mix well.
Fold in shredded cheddar, dill, chives, and red onion if using.
Add cucumber slices and gently fold until evenly coated.
Cover and refrigerate for at least 30 minutes before serving to allow flavors to meld. Stir gently before serving.