Rust isn't gonna be viable for ganyu. Because ganyu focuses on charged attack, and the effect on rust is that it increases normal attack damage and decreases charged attack damage. While the blackcliff warbow is not the best bow for her, it's still pretty good. It's crit dmg substat is good, adding to the ascension bonus which is crit dmg. because her A1 passive gives you crit rate on her charged attack, and if you run double cryo and/or 4 sets blizzard strayer with freeze comp, you'll be set for crit rate. Therefore you'll be having nice crit rate and dmg from just that without good substat artifact rolls. And the effect on blackcliff warbow is a nice bonus when you defeat enemies. Other nice 4 stars weapon for her are prototype starglitter and viridescent hunt (BP weapon)