It skipped my mind, but craft Alice's Rainbow Greaves if you don't have them, it's really late to make them but they're still one of the best things she can use by granting all elemental resists including MYS. Even when you're using the massive resist accessories (resist all magic, weak to phys, and vice versa) the resistances it grants will counteract the induced weaknesses and help you out, so you use them basically forever 'til the mega rainbow greaves open up at the very end.
HOWEVER, that will only help Alice. If you want it for Aya you'll need to farm one of those drop rings as RegalStar said. THE FINAL PHASE STILL CASTS MINUS, so you still need Alice's greaves. Also please equip either a big permanent resist or mega petrification resist item on Alice, she'll need it. If your first party is fighting Phase 2 you might also consider earth resist in any spare slots, because Stone Shower hits everyone and hurts. Sanae's MDef buff can work instead, too.
I'd recommend shifting Sanae to party number one. Having both Sanae and Byakuren in the same party severely gimps the other, and in streams/videos+both my own playthroughs I think that's how I generally see it done (sanae party one/byakuren party 2, that is), unless they were attempting lunatic form final boss. This will make your first party survive a lot better and deal a lot more damage.
Keep in mind this means you do want Amplifying Scroll and multiple heals on Byakuren (DEFINITELY the permanent healing one) and some additional avenues for support. Nitori commander as you're already doing works for her 3-bomb- Satori in the opened slot in the main party is also very helpful if you have her relevant support skills like Reflect Wall, Area Heal, Starlight Barrier, etc. Satori is also really good in party #1 if you have mad thunder, which deals sick damage on Anastasis and solid dps for phase 2, and Shield Wall works if you really need it on Anastasis turns. Really, considering you're a fan of Satori, I'm surprised you dumped her in the commander slot, she's VERY good in either party if she's learned the relevant skills. (Your new commander might be Reimu, if so, use that exorcism bomb on Lithos turns- 40 divine barrier is no joke and Mokou's fujiyama will melt her)
The only thing is that you won't be able to revive in Party #2 unless you put Mokou in the commander slot. This isn't necessarily a problem, Nitori's a good commander too, it's just something to consider. You might also want Byakuren to bring Perfect Resist and cast it on Youmu so she'll resist Vortex, because Youmu losing manussya+patk+dark enchant sets you back several dangerous turns. (It's nice on Alice if you get a spare turn for it) You also want to make sure you've got 6 bombs and Moon-Reflecting Satellite Slash (lv60) on Youmu, but 7 or even 8 bombs means you have spare spellcards you can have Youmu cast in phase 2, which you might need (or otherwise gives you time to buff Youmu before the final phase). Getting stances to 5 pow grants a bomb.
(There's also a 10% MYS enhance amulet with 75% MYS resistance that drops from Golden Scarabs in stage 7 but that comes with some physical weakness so I can't recommend that.)
Uuughhhh I'm having flashbacks from my second playthrough where I got every item drop. These things have Yukkuri encounter mechanics. Each time you encounter a scarab on a specific map, it's encounter rate goes down, christ that drop took me literally 6+ hours I was so unlucky ughhhhhh. It might have been 10 hours but that was years ago. After I farmed down it's encounter rate on every map to where it took 5+ minutes of constant running to encounter one, things got really painful.