I want a moment to complain about how BAD Bond Requirements are in this game. It takes hundreds of thousands to unlock the Bond CE, getting meager quartz along the way IF your servant is high rarity.
And often the Bond CE effects aren't helpful at all (likely why the Grands can choose that or a 50% charge)
And sure, the 11 onwards giving 30 quartz is actually significant but again, gated by over a million bond.
And then there is the coin system that outright depends on it. This sucks. I'm 80 floors in, all my min-maxing barely put a dent on the grand scheme of things. Sure, some Bond 5+ servants reached 6+ but SOME. And that's it.