According to Extremists guide, between 33 and 35 Reviane is supposed to show up outdoors. I've been visiting every outdoor zone now, travelling back and forth and she just doesn't show up. Is there something I can do to fix this?

Tried resting or doing some Harper-related quests?

Yes, you need to do some Harper plot and quests for the romance to go on, IIRC.

I've killed Galvanyr already. What else is there to do?

Visit the Spectral Harpists upstairs?

That's something you have to do in order to get the Galvanyr quest in the first place :)

but it eventually worked out for me anyways, de'Arnise lands seem far more scripted_encounter friendly than the areas I tried before.

Well, Revianne appeared in my current game right after the 'thug' encounter which happens if you rest in the wilderness. I have done other Harper-Jaheira associated quests before.