It would be better to farm Ultimate raids because you also get regalia+fragments, and a chance for Assault SRs. You want to fill out your whole weapon grid with assaults. Regalia are for soul weapons which you will eventually need. Eidolon orbs should not be your priority, but instead a bonus addition when you are farming the 2 things I've mentioned. Also, it might be worth farming other elements, because who knows if you'll eventually get the best himes for said off-element. You'd need multiple himes for multiple elements for tower events. Make sure however, you are focusing on one element first which in your case is Light. Spend 50-60% of your resources on Light, and the rest split between all other elements so they will be relevant when needed.

Also, a tip to getting extra eidolon orbs would be, sell your event eidolons, mainly the elemental lilims. Those are horrible. Very low stats at max(hp+atk), requires 3 elements in main team, meaning the first 4 hime+soul, and usually nobody wants to choose them as support eidolons. That means you will end up getting even fewer gems everyday. Level a copy to 40, get the scene, and trash them.
As for non-lilim eidolons. If you already have a MLB eidolon of the same element(with Elemental ATK↑ and NOT Characters' ATK↑), unless you need the "stats" you could sell them too. Only time you'd keep is if you are unsure if their special effects are worth keeping, or you need them for stats. For example, let's say you already have an MLB Light Eidolon(Barong). You get Nandi and Light-Lilim from event... Nandi gives 2622 total power, Lilim only gives 1992. Keep Nandi for stats, trash Lilim for orbs. Only lilim ever worth considering keeping is the Phantom Lilim, just because you probably don't have any Phantom Himes... although you'd still want to trash her for orbs.
But you are likely going to be keeping event Eidolons for a while until you start finding the need to replace them for said element.

None of the past eidolons are worth it, except for Tiamat who has very high stats and her summon effect gives a barrier. You can place her in sub-eidolon slot for non-water team. You don't need to rush grabbing other eidolons from the shop because future events will give you eidolons that are sufficient enough. You want to hold off spending eidolon orbs on off-elements until you get a decent hime team-comp and high enough assault level in your grid.

For draconic eye shards, you get the most from Union Events. So make sure you join an Union that is reasonably active.

tl;dr Get in an Union, run all Ultimate Raids for assault weapons+regalia, orbs are a "bonus" not a priority focus, SELL ALL LILIMS after getting their scene.