For the most enduring single player experiences, you really should give some grand strategy games a try, like older Total War entries, Crusader Kings, Civilization, Swords of the Stars, etc. Stuff like Factorio or Cities Skylines can also become addictive, but none of these games is action oriented.

Also, Age of Empires 2. I haven't played 4 yet, but I think it speaks volumes that, at least on steam, AoE2 has more players at any given time than 3+4 combined.

Ziggurat might be an interesting pick, as it's a roguelike FPS with magic weapons, though it might feel super neutered compared to Ultrakill. Risk of Rain 2 is 3rd person and roguelike, so every run you start from scratch and enemies will keep spawning at certain intervals, but it's a fine shooter

Fallout 4 might have a rather clunky shooting, but if you get into it, you can spend many, many hours blasting a variety of enemies, finding all sorts of places and weapons. Skyrim is a close second, while better played in 1st person, it's medieval sword and sorcery.