Honestly if you want recasting materials without being insanely bored I'd say do the kill X number of Yakuza missions. Once you're done killing them all you can pick up all the swords to extract them at the blacksmith. Plus doing it this way you can master styles while you're gathering recasting materials. Though I can't remember if doing Ogre Isles on Harakiri frequently gives Dojima or Ground steel. When I did them I only remember getting scrap metal.
But for the weapons DLC, they honestly are well worth buying. Maybe not all of them, but at least one of them. Especially if any of the weapons have parts you want to use for your custom weapons, will save a tremendous amount of time and effort going through hard mode trying to get the weapon to be able to extract/reforge it until you get the charms you want. Personally I'd advise the one with Kotobuki and Moro's weapons. Can't remember what else it has, but they're extremely nice looking swords. And extracting them on Harakiri they almost always give dojima steel/ground metal.
That's a great way to get materials, yes. I also like to just hang out in the Back Roads, kill every armed person you come across (samurai, ronin, yakuza, magistrate, deamonscale, ect) and pick up their swords. Quickmove back to town when you get 10, get your materials, then go back to the Back Roadsand do it again.
With a "grab" type move (such as the Chu-dan move where you sweep their legs and then stab them in the gut while they're on the ground) you can kill them from behind in a single hit before they even draw their weapon (or close to kill, on harakiri)
The main merit to grinding the yakuza mission (from my perspective) is that it's a multi-tasking dream.
you kill 25 yakuza in town, where dojima is located. Dropped swords will not disappear until you zone. Bodies will, but not swords. In fact, in some missions where enemies spawn infinitely this can cause the game to glitch, slow down, and crash. But 25 is fine. You will most likely get scrap metal from each sword on easy an normal. On hard it will be common to get ground metal as well.
You earn 1250 ryo per mission, plus you can sell the items. The yakuza tend to drop either red peppers, sake, lucky bamboo grips, or a small amount of money. Peppers sell for 250 ryo, sake sells for 150. It is common to get at least 3500 ryo per mission after selling items.
I level up styles why I do the missions. Considering the enemies are fairly weak, regardless of difficulty, I can easily learn moves with annoying stipulations: I.e, "while sitting on the ground", "with a base move while healt is less than x", "parry or counter", etc.
Since I'm fighting, the constant wear and tear of my swords gets me that much closer to unbreakable without having to isolate grinding durability. It is extremely unlikely that you will wear your sword enough times to reach unbreakable... But it gets you closer, and that means less time spent doing the isolated grinding.
It keeps the crime rate great!
So in short, one mission gets me materials, money, exp, and wears durability on swords and keeps everything in stores and services cheap! Materials and money at the same time is already an awesome benefit. I bank most of the cash to use when upgrading swords, or buying clothes and accessories. Also can turn the cash into chips at the casino instead of playing poker to buy accessories and moves scrolls. Leveling up styles at the same time means I'll ultimately have to buy less moves scrolls to max out styles.