Saras exact changes:
35 elemental -> 40 elemental
25 BG -> 30 BG
30/20 DATA -> 45/30 DATA
It's probably the best buff of the group imo. She was never too horrible before, and is now just better all around. The DATA buff is enough to make it pretty much guaranteed with accessories. That said, still not really worth mtix her.
Hades buff was fairly lack luster, the damage cut is nice (especially the duration), but she's still too reliant on debuffs to do her other thing so... eh.
Osiris got better at healing, which is fine, but the additional BG gen is barely noticeable and on too long of a CD (also taunt isn't that great). If this means she won't get an AW later, this is more of a disappointment than anything else.
Bastet is... huh. I guess she's usable and not just a stat stick in reserve anymore, but... still not good enough to save up jewels/spend on IMO.
Jupiter is better... but Thunder just really doesn't have room for her, sorry.
Regarding how to use your regalias: I think flb weapons is a slightly better option than mlb a soul weapon, because the additional skill levels and often additional new skill is pretty big. That said, it depends heavily on the weapons in question ofc. Also, you should really do both asap anyways so it doesn't REALLY matter that much in the end.
For soul weapons: Herc is fine as a place holder until you can run Shingen, even if you don't super need the def down from her weapon, and more so if you can replace a debuff slave with a more offensive one by doing so. That said, if you really don't need her, the alternative to those two for vets is Arthur, who is kind of a cross between the two, one that can still hit hard but also can support your team a bit (and very good during burst time).