Nahh, if I were you I'd use 산티아 ㅌ sounds more like 't' so it'd be 'Cyn-tia' 신시아 = Cynsia 신띠아 = Cyn-ttee-a I'm not an expert at Korean here, so I'm not 100% sure I'm right. Maybe someone can give a better answer?

Most Koreans can not pronounce 'English /th/ sound' exactly because we don't have that kind of sound in Hangul. However "신시아" is often found in most English-Korean dictionaries and it's very comfortable for us to pronounce it as "시". ^___^

보통 사람들은 신시아라고 할 듯 그런데 그냥 이름 물어보면 스페인어로 발음해도 될텐데