Checked the thread again and the update OP. I gotta say, you've done really well on it. You even made a chart which I don't see often particularly on the item drops. What are your traits and specifics on getting 500+ luck? have you noticed any differences regarding stages? obvious differences between Twilight and original?
My Luck setup was:
Daiba Washi primary, Itokuri secondary @Lvl50 – +70+42 (that's better than the other way around)
max. possible bonus from titles – +90
2 Yata Mirrors – +60x2 (I have one with set requirement -1, so don't need Yasakani anymore)
Swallowtail set + Kodama bowl – +75 from set and inherited luck on most pieces ~+100
I used this to get the smithing texts in the last DLC (without the Yata mirrors obviously), so it's not really a luck build or anything, just something I had available.
The data comes from a single loot spot in “The One-Eyed-Dragon's Castle” that always drops a rifle and boots, since you just run past everything the set doesn't need (nor has) survivability. I wouldn't claim that the drop rates from this spot generalise to all “high quality” loot spots, or overall ethereal drop rates, it's just a point of reference.
I also killed the first Bandit in Isle of Demons WotW till I got an ethereal from him, that took around 150 kills without any Luck boosting in particular.
And currently I'm trying to get an Ethereal Monohoshizao with Susano bonus from the Ganryu mission and get an Ethereal drop roughly every 4 kills with +300 Luck, Luckbringer and Ochoko boost (I quit the game, if there is either no, or a useless Ethereal drop instead of finishing the mission, to save Ochokos)
So that's a bit more data…
edit: I finished my Monohoshizao farming. From 130 loot attempts 62 Ethereal items dropped (sometimes more than 1 ethereal item dropped and those were counted as such. I've got up to 3 ethereal items per kill)
Anecdotally I'd say that Higher Level mission yield more Ethereals and Twilight Missions seem to be better in general, but I didn't gather any hard data on this, if you start taking notes after every drop you won't get anything done in this game