There will be something called eidolon shop implemented soon (expected around August). You will obtain orbs as well from selling your eidos starting from that point & those orbs can be used to buy various stuff like Half Elixirs, Seeds, Books, weapon/eido LB substitutes up to SR rarity (aka bricks), previous raid eidos & a single copy of each Kaiser (pricey af).
As for SSR weapon, I guess you already have the answer.