The Nightreign character with Sekiro mechanics (Executor) is actually the best solo character because he can tank any attack with a perfect deflection. This also makes him great if you want to play coop because once you can solo a boss you can just carry any team. Its really fun.

Randos can be tough to deal with, sometimes they leave if you don't do exactly what they want you to do, or they suck ass, but usually this isn't too bad. Once you get into the endgame, Deep of Night mode, then yeah its better to find a coop community with a better player pool. Other than that, Nightreign is fun solo, esp with Executor.

I have about 300 hours in both Sekiro and with Executor. But I'd def recommend Sekiro because it has one of the best stories in any From game, is beautiful and slick and very replayable for a single player experience.