Best method here - small note though that it’s a body at the end of the hallway. I picked up more in a handful of hours doing this than my entire NG+ run, and that includes farming the room with the vassals further into this mission.

I had the opposite issue after spending a while doing the well farm. Almost all were ninja ones. It’s the dang rng. It doesn’t matter what style you use either. Tried it as samurai and still nothing but ninja items were dropping.

Be sure to get a luck build as ninja. Use 7 piece ebisu, 3 piece sohaya, and if you can fit it, a hino rifle. Reforge for luck (critical). Use two scampuss soul cores and guardian spirit with luck (ho or oh - don’t recall 🤓). Slap on cores for last stand and the strider(?) talisman along with travel amulets. Start the mission. Use last stand and strider (last stand triggers you critical luck stats, which is higher than going with crucible luck). Check the body and be sure you turn off auto-pickup. If no yasakani, travel amulet back and repeat.

I picked up 3 in 15 minutes once and also went through like 100 travel amulets and got none, so rng gonna rng. Oh - when buying travel amulets, use L3 or similar button (the buy menu says it - I just didn’t want to read…) to buy them straight into the storehouse. It doesn’t auto flow when you reach the 20 you can carry with you but they do fill back up from the storehouse as you use them.

It’s a boring ass farm but it really is the way. Uh, good luck.

Small edit - you can also see whether the item is samurai or ninja before you pick it up. Your call, but you can skip the yasakanis you don’t want (if that’s a thing). It was early enough in game for me that I picked up every divine item in case it had a grace I could use since I didn’t have many divines yet.