A 'bit' more damage would be wrong. Rs can do double or triple damage of off element SSRs. What matters is your weapon grid and main/friend eidolon. Since your weapons arent leveled and many incomplete, of course it would see Osiris does more.
Zeph also does a c-frame debuff, meaning youll take less damage and need less heals to begin with and you dont need Osiris to heal when you have Ithaqua, assuming I'm not mistakening her for another kamihime.