Sonja sits next to 3 phonetic neighbors — Sonya, Sonia, and Tonja are the closest matches by sound. Most share Sonja's Russian-origin cluster, so the rhythm and vowel shape carry across the list.
You searched for names like
"wisdom, skill"
"rapids, flowing water"
"true image, sacred icon"
"ruler of the estate"
"fame"
"noble one, of noble kind"
"city of history and culture (as a name)"
"lily flower, purity"
"horn (from Cornelia)"
"old age (Norse mythology personification)"
"flowing stream"
"alluring rock, murmuring rock"
"worthy, powerful"
"light, torch"
"pure, innocent"
"God is gracious"
"free woman"
"battle woman, warrior"
Similar sound: These names share phonetic patterns with Sonja — similar vowel sounds, consonant clusters, or rhythm.
Shared roots: Several names on this list have Greek origins, giving them the same cultural heritage as Sonja.
Rhythm: Sonja is a 2-syllable name. Names with the same syllable count tend to feel interchangeable when you're narrowing your list.