No place is perfectly safe, but I feel safer in Barcelona than I do in most cities. Petty property theft can be an issue (although you can prevent most of it by vigilance) but personal threat or harm is rare. FWIW, I am not a fan of Las Rambla - the lower end can be a bit shady, but closer to Placa Catalunya should be fine. And I would take the metro over walking for an hour. JMHO.

IIt's not even a very interesting walk from Sagrada Familia to the Rambla, definitely better to take the metro. The metro is fine, keeping a close eye on your belonging is the most important thing.

And btw it is "Sagrada Familia", asking for "Sagrada" outside these forums will get you nowhere, the word means simply Sacred or Holy

Everyone has their own tastes, of course, but I completely disagree with the advice given earlier. The Eixample district has dozens of modernist buildings on its streets, most of which are private - hence, not visitable - but you can admire them from the outside. The path from Sagrada Família to Les Rambles is filled with examples of such buildings. Therefore, if you're into architecture and art, I do recommend a stroll from the basilica to Les Rambles. There are some iconic buildings (e.g., La Casa de les Punxes, Palau del Baró de Quadras, or Casa Garriga Nogués, among many others), but many others, less known - over a hundred - offer exquisite details on their facades.

The Eixample district is divided into two parts, "dreta" (right) and "esquerra" (left).

https://ca.wikipedia.org/wiki/Llista_de_monuments_de_l%27Eixample_de_Barcelona_(dreta)

https://ca.wikipedia.org/wiki/Llista_de_monuments_de_l%27Eixample_de_Barcelona_(esquerra)

As per safety, I concur with the above: no place is perfect, but this area of the city is "quite" safe for a walk "after dark", moreover taking into account we Catalans are very outgoing so you're bound to find people in the streets until quite late.