The first title that jumps to mind is 'The Warehouse' by Rob Hart. It shares that eerie feeling of a corporate entity swallowing up every aspect of life, treating employees like parts in a machine. The company is called Cloud, and it's basically a hyper-dystopian Amazon, with live-in workers and this suffocating, cheerful corporate culture that masks something deeply sinister. It nails the unsettling vibe of 'Severance' where the workplace isn't just a job; it's a totalizing environment with its own rules, secrets, and punishments.
Another one that's less about the literal sci-fi but captures the psychological paranoia is 'Then We Came to the End' by Joshua Ferris. It's set in a regular ad agency, but the atmosphere of collective anxiety, rumors, and the feeling that management is orchestrating something the workers can't comprehend really echoes the outie/innie dynamic of not knowing what's really going on. The mystery isn't a secret floor, it's layoffs and unexplained decisions, which honestly feels just as terrifying in its own mundane way. I found myself getting just as creeped out by the mundane office politics as I did by Lumon's perpetuity wing.
For something with a more direct, tech-thriller conspiracy, 'The Circle' by Dave Eggers fits. It's about transparency culture gone mad, where a single tech company demands total integration of life and work. The mystery unfolds as the protagonist realizes the utopian vision is a cage. It lacks the surreal, retro-futurist aesthetic, but the core of a benevolent-looking corporation hiding a dehumanizing agenda is spot-on. Reading it, you get that same claustrophobic dread of being watched and controlled, just through a screen instead of a severed brain.