Dunno much about thunder, but if you're struggling with debuffs, Hercules with MLB axe can often be better than Shingen (e.g. if using Herc moves you from 30% def down to 50% def down, that's actually a 40% increase in dmg, which can be as good as Shingen reducing your burst cycle from 5 to 3T, depending on what your DATA and exceed looks like.)
Not sure what your grid looks like, but if you have to drop two MLB SSR weapon to accommodate 2 LB0 SSR just to activate a phantom weapon, it's usually not worth it, especially if your phantom weapon itself is LB0 (so you're effectively dropping 3 MLB SSR.) If you've done the math and know exactly what you're doing, sure, go ahead, but I would caution against burning eyes just to get a phantom grid running full of LB0 weapons. Also consider using MLB SR instead of LB0 SSR, they're often better - burn orbs to LB ones that are good, at least orbs are easier to farm back.I'm also considering pushing to get 2 more Thunder SSR glaives so I can make use of my one and only Phantom weapon in Thunder.