Thaddeus sits next to 4 phonetic neighbors — Theodore, Tobias, and Matthias are the closest matches by sound. Most share Thaddeus's English-origin cluster, so the rhythm and vowel shape carry across the list.
You searched for names like
"courageous heart"
"battle chieftain (Cerdic interpretation)"
"twin"
"one who lives near the fens or marshes"
"one who catches fish"
"continuous improvement"
"three, the third"
"enlightenment, calm clarity"
"born at dawn"
"spear, lance"
"foreigner, Welshman (Celtic speaker)"
"of the Lord's day (Sunday)"
"sea warrior"
"spiritually reborn through baptism"
"little red-haired one"
"nobleman, duke"
"high stronghold, fort on the headland"
"rocky land, stony ground"
"God-like"
"throne of the eye (possible etymology)"
"the cleansing one"
Similar sound: These names share phonetic patterns with Thaddeus — similar vowel sounds, consonant clusters, or rhythm.
Shared roots: Several names on this list have Aramaic origins, giving them the same cultural heritage as Thaddeus.
Rhythm: Thaddeus is a 3-syllable name. Names with the same syllable count tend to feel interchangeable when you're narrowing your list.