There is no point wasting orbs in the eido store on any other eidos besides the kaisers. The kaisers have the best on-use summon of any eido, and that's the main reason why it's beneficial to have that for every element (or at the very least, the ones you use). Some people might advise you to first get something like another MLB tiamat for your eido grid stats, but eido orbs for kaisers are already slow enough to farm as it stands, that ultimately all you're doing is delaying yourself from acquiring the kaisers even longer.
As for whether dupes or new ones, depends on how strong your teams are and how often you use the specific elements I guess. While I have no idea about your teams, the fact that you have Belial could make the fire kaiser a good choice. Then again, thunder would be good for the next tower coming up as well. So that's something you'd have to decide based on your teams and what you use.
Well to me, rank doesn't mean much either. There are plenty of rank 100+ ppl that are just as weak as babes and wouldn't do much before dying on AAB.