>>938022465gf sister shower poster here. My setup is an old phone of mine (screen issues but not too far out of date so still a decent cam) that I sanded and spray painted black to get rid of the shine, everywhere except the lens. It's resting above the fan grille, which, at least the ones I'm used to, you can just pull down without any tools and then clip back up. People rarely look up (especially when in the shower and that'll just mean getting water in your face) and I was surprised at how hard it is to see even when I know it's there.
It's plugged into a power bank so it can record for longer (~8-10hrs), make sure you turn off h265 (or whatever it is) video compression in camera settings, at least on mine it made it overheat and stop recording after about 30 minutes. I remote in with teamviewer to make sure it's lined up right and recording, then turn the brightness all the way down and enable extra dim (or you could use a 'screen off' camera app).
Have heard Blink cameras mentioned a bit, they look pretty easy to hide but not sure how reliable they are or what the image quality is like.