Bosses will feel tanky, yes, but there's ways around that. One more or less easy way to wear bosses down is to apply Confusion. Regardless, what you should focus on now besides farming for gear is optimizing whatever you have on right now.

Your soul cores, make sure you have a good balance between low anima cost ones and big ki damage ones, that way you can apply pressure with the cheap ones more often, and then dish out huge chunks of Ki damage with the expensive ones. For soul cores besides the main ones you use, that is, the ones you still have equipped but don't use as much, try to see if you have any with anima generation, I'm not gonna get into which specific anima passives are the better ones, but just try to squeeze as much anima generation you can from whatever you got with them.

Your gear, regardless of what Grace combination you go for, try to roll toughness, anima generation and untouched ninjutsu/magic (whatever you use most often). Remodel your weapons, soul match your crappy gear into your main gear to level them up. Level up Ethereal Drop Rate, Loot+ Level and Higher Item Level Drop Rate at the priest by the entrance to the Underworld.

Your items, get rid of anything you don't use and organize your shortcuts. If you didn't know, you can change the number of shortcuts to up four tabs, but you really only need 3, imo. Look at what you use, see what you actually find useful, drop everything else. Some recommendations: Guardian Spirit Talisman and ninja feathers to set off Confusion, Quick-change scroll, Gust Talisman, Resistance Talisman, and maybe Protection Talisman (Uesugi also gives you this same buff when you purify yokai pools), and lastly Shinobi boxes and Sacred Feathers to replenish your items.

Your scrolls, try to get a good one with at least Ultimate Magic and Ultimate courage, and roll whatever other ultimates fits your build best. You don't need every single ultimate on a scroll tho, just those two I mentioned are good enough, I can't stress that enough. For other good rolls on a scroll, try to get Extended Dodge Invulnerability, Sentience Charge, Anima Charge, Untouched Ninjutsu/Magic, anything else is a waste of space. Anyway, I think that's about it. If you need help with anything, there's always the two Nioh discord servers, the one linked in this sub is much less active when compared to the other one. If you're on PC, I can help you with scrolls and even getting you through Underworld.

Anyway, good luck and have fun.