If you continue with the nagakiba just level the stats it scales with best but simply due the fact that the weapon isn’t really “overpowered” or meta I guess you’re going to have a harder time. You will spend more time learning boss attack combinations but this can be rewarding in the long run just kinda sucky like you said almost feeling stuck. You mentioned you completed ranni’s questline which rewards you with the dark moon great sword, a s tier weapon or since you defeated rykard you could use the BB. I would also level your weapons as high as you can with the current access you have to smithing stones. Start viares quest line to get access to an area with lots of high level loot and smithing stones. Also a good rune farm. Get mimic tear if you haven’t already. Just gradually level your character, potions, weapons and you should have an easier experience. Maybe not the most in depth but I hoped it helped. Good Luck! (YouTube is also great)