I just beat
Sekiro!...
FINALLY! 212 hours I spent playing that game

About 80 hours of that was just redoing the first 3/4 of the game after my save got corrupted.
Got the Dragon's Return ending, because after all that, I was going to make damn sure I did everything. Not real clear what happened though. The Divine Child is somehow pregnant with a baby Kuro?? And she and Sekiro travel to China.
Sword Saint Isshin was tough, but nowhere near as obnoxious as the Demon of Hatred. Took me about 2 1/2 hours to beat Isshin. Came within one hit of beating him a couple times and choked. Honestly, I'd say even Owl (Father) was harder. Then again, both the Demon and Owl (Father) are optional.
Honestly, the only really hard part of Isshin was the second phase when he pulls out the spear. He has ludicrous range (not even counting the gun), and the timing on his attacks is very tricky and hard to discern. Phase one you can basically just run around until he starts charging up something and then get behind him, and phase three you can just abuse the lightning reversal.
Now of course, with the recent update, there are "Inner" (read: super) version of Gennichiro, Owl and Isshin. Haven't attempted any of that. I'm debating whether to go for the Shura ending, just so I can say I did everything. Been putting off Horizon Zero Dawn to finish this game though, so maybe I'll switch to that for a while.
Man, that was exhausting...