If you're unsure about the difficulty, yeah better wait for a sale. Sekiro was my first soulslike game, but I soon learned to love it. The combat relies heavily on actual parrying rather than dodging (like in Dark Souls). Once you get the hang of it, it is soooo much fun. Some time after Sekiro, I actually also tried Dark Souls Remastered and I gotta say, I don't understand the discussions about which one is more difficult, Sekiro or DS, because in my oppionnion there is nothing to discuss. Sekiro is definitely harder, because in Dark Souls you can grind lower enemies to get stronger and you can even have other players help you win a boss fight. LOL! In Sekiro you have to kill every boss all on your own, and defeating bosses is the only way to actually get stronger.
In the beginning, you can go quite some way with the stealth mechanics of the game, but eventually there are the first boss fights you will have to pass to continue the game (some fights can and should be skipped at the beginning). If you succeed one of the easier boss fights, you will probably be hooked to the combat system and continue the whole game.
I played it with controller because it feels very natural this way, but I saw numerous speed runners use mouse and keyboard.
Arkham Knight suffers severely from the "too much todo in an open world" phenomenon, i.e., the Ubisoft Formula. I never finished it for that reason. It's just too much content to keep you hooked.
Sekiro's combat differs significantly from Dark Souls', as stated above: It relies on parrying rather than dodging. That being said, I died a lot more often at the beginning of Sekiro than at the beginning of DS.
Regarding the atomic catastrophe, you are probably talking about Atomic Heart, which has not yet been released.