-45% Def down is fine. I'd definitely use either Herc with the Axe or Shingen with Encourage Inspiration. With both Osiris and the Inspiration, the Dark team is actually decently fast. Atk down isn't nearly as needed as Def down. After you get Provisional Forest, the whole goal is to kill fast. If you reduce the rounds dramatically in a combat and burst hard, that's the same as ATK down.
You'll probably want to stick with Ambush on the Light Team as -30% DEF down is fairly light. OTOH I'm currently running a Light team similar to yours with Artemis instead of Tsuku and I only have -35% DEF down. Using Encourage Inspiration to just hit faster seems to be working out fine for me (but I actually can't use Ambush since Artemis has an A-frame DEF down).
The Dark team is also fairly close to my Dark team, but Dark Amaterasu is a great addition. You can definitely try Shingen with that team as you have no use for Sniper Shot. I'd swap Samael in for Satan Awakened for GO actions, but that is a solid team. I also fully skill leveled Chernobog and Osiris' weapons. They are Assault/Defender. Even at level 50 you're getting more stats than any level 85 SR in your grid. Even Samael's Hammer, Dark Amaterasu's Mirror and Thanatos' Necro Joker are damn nice. If you skill raise all of those, you'll come close to a fairly strong grid (probably a lot stronger than your Light grid) and you can run that team as confidently as the Light one. Having 10 assault and 8+ defender skills at full level works wonders - even if the base hit points and assault are not there due to only having the weapons at level 50. It's something Light grids can't as easily do (as Tish's weapon currently sucks and a lot of the other ones are not Assault/Defender combos).
Dejnov.
P.S. I should also mention that Frey's, Eros', and Atum's weapons are also Assault/Defender. Raise those and stick them in your grid over any SR and even single skill SSRs or one's that don't have both Assault and Defender. The percent bonus will overcome the 75 levels of base assault you're missing.