Was he wrong to forbid his fiancée from sending religious messages that encourage good deeds, as a pre-emptive measure to ward off potential evil (sadd al-dharā’iʿ)?

We believe you have done well in blocking the means (Sadd al-Dhara'i'). The fiancée is a non-mahram to the fiancé. Exchanging religious messages between non-mahram individuals, even if not forbidden in itself, is a means to other undesirable communication. Therefore, blocking such a means and keeping away from it, as a precaution against doubt and to avert temptation, is an excellent form of piety (wara').