I'm skeptical of the entire premise. Books that try to clone 'Fifty Shades' often replicate its worst qualities—stilted dialogue, questionable consent frameworks, and thin plots outside the bedroom. Maybe instead of looking for a clone, look for a book that does office romance and explicit tension well in its own right. Tessa Bailey's 'Fix Her Up' isn't an office romance, but her 'It Happened One Summer' has a similar dynamic between a sea captain and a socialite—clash of worlds, explicit scenes, great tension.
Sometimes the best fit isn't in the same subgenre cookie cutter. Branching out might lead you to a book you love for its own merits, not just because it reminds you of something else. The heart of the request is 'forbidden, power-imbalanced, explicit tension,' and that exists in many settings.