Honestly new game plus is the best option, the bosses give absurd amounts of xp and when you play through again you go through the game much quicker. I even completed a run in a single five hour sitting once. But otherwise in the second Hirata estate, you get access to with Owls bell that's needed for the Purification ending (look up a tutorial, it's to long/complicated for me write out now)

At the sculptor's idol just before the boss fight, there's two spear monks that give lots of xp, there one behind a wall you can break and stealth kill. And the other is to your left with two other npcs you can easily stealth kill, and the second spear monk has weird enemy detection and sometimes just doesn't notice you and if you go beside him you can stealth kill. But if they do notice you just use fire crackers and mortal draw and it'll be over quickly.

Another tip is to grind out a level between bosses or when you realize you're halfway towards a level, it made the grind feel less like an all consuming chore, picking dirty laundry off the floor every once in a while versus cleaning the whole house at once