Either you have Managarmr and need light eidolons in your sub slots
Or you're lacking a 40%+ thunder eidolon and thus Phoenix can fulfill that role until Tartarus

Although as far as potentially rolling Managarmr goes, there's no harm in holding onto a maxed Phoenix for the time being, until post... Nandi or Adou?
There's no difference between realized and unrealized 750 orbs until you've actually hit the 10k for a kaiser dragoon. Unless you're really desperate for inventory slots, but that's really hard to do with eidolons.