Si sits next to 4 phonetic neighbors — Cy, Sai, and Sy are the closest matches by sound. Most share Si's Chinese cluster, so the rhythm and vowel shape carry across the list.
You searched for names like
"God has heard"
"iron-willed, strong"
"army of the people"
"skilled metalworker"
"God is gracious"
"alder tree dweller"
"associated with Saint Spyridon, miracle worker"
"Dark one; descendant of Ciar"
"sun"
"house of God (possible Bethel derivation)"
"Ford by the windy place; a river crossing in a breezy location"
"field settlement"
Similar sound: These names share phonetic patterns with Si — similar vowel sounds, consonant clusters, or rhythm.
Shared roots: Several names on this list have Hebrew origins, giving them the same cultural heritage as Si.
Rhythm: Si is a 1-syllable name. Names with the same syllable count tend to feel interchangeable when you're narrowing your list.