I don't think you should be worrying about the best weapons at this point. These vary with every patch, now with the DLC coming, there will be always weapons to look forward to. If you're still playing the first play through, so not NG+, just use the best gear that drops at that particular moment. The highest level gear is lvl 150, so up until the point enemies start dropping this, you can use whatever you'd like. After this point and while playing NG+, you need to boost that lvl 150 to lvl 160 with soulsmatching divine gear which will raise +x after lvl 150 up until 150 (+10) (160).
Btw. I don't understand what you mean by kusarigama earth damage at lvl 623? ) is your character level 623?
And what do you mean by adding an element buff with magatamas? You mean like fire damage to your sword from magatama accessory? Those accessories provide only resistance to elements, don't buff your weapon.
Btw. this game is so extremely complex, that even though I spent a lot of time reading around it, I still just seen the tip of the iceberg and still not fully understand the game mechanics. Thats why I ordered the Future Press Nioh Guide, which comes out in April. Can't really go without that if you want to play the game correctly.
The Kusarigama I have has a damage level of 623 with earth damage buff. My character is now at level 123. I've just been running around leveling up as I go. I'm not on NG+ yet
Ok, that makes things clearer. the damage 623 you have on your kusarigama is not "level", its simply the damage it makes. The weapon itself has its level and since you're lvl 123 currently, it will be somewhere arond that. So as said, as far as you're not lvl 150 so far, you don't need to worry about best gear and use just whatever fits your playstyle best at the moment, or what you like the most at the moment.
I haven't. I run a ninja-build, based on dexterity / skill running with kusarigama / katana and wearing the head of the iga set (full 7 pieces). Farmed Hattori Hanzo a whole evening and killed him more then 200 times to get all of it in the dojo mission
It really depends on what your build is focused on, but as said, you got time to figure out until you reach lvl 150. Thats the point when you should start considering the best set to use. Until that, feel free to experiment and try out what suits your playstyle best.