From what I've gleaned here and there about Sands drop rate :
Ennead and Diaspora 6D have a ~0,1% chance to drop
Diaspora tier seem to have a ~1% chance to drop you can see this guy getting a 1 for 5 ratio of sands vs schrodingers.
SUBHL seems to also be a ~1% drop chance, but I'm not seeing a lot of data for it, though it would make sense considering it's the a 8 stars raid like the diaspora tier.
So yeah, it's actually very inefficient to farm those while punching dragons and ennead, even using 2-3k BP doesn't guarantee you a sand drop... It is the fastest however, and nets you useful mats overall. Diaspora tier is better to farm efficiency wise, but those raids can be pretty dead if not done during JP primetime (only Diaspora still fills at any hour), and can't be simply leeched.