But when I went back to the store they'd changed 'seasons' or 'styles' and the same items were no longer available
Stick to classic designs from classic brands and you won't have that problem. The basic white dress shirt/blouse for example works for all occasions and if you find a fit that works for you from an established brand you'll never have to worry about fit again (until you change size).
I buy 2 pair of Levi's 501 on sale each year for ~$35, a single brand/style of underwear, single style/brand under shirt, single brand/style of athletic socks, 2 pair minimum of any style/design of non-athletic socks, etc. Button up shirts purchased as needed (trying to limit to 5-7) from same brand, and all matching jeans. I can blindly grab first item in each stack and have consistent results. haven't felt need to simplify any further. I try and stock up on the socks/undergarments during sales, and then replace in drawer once showing signs of excessive wear. so far so good.
Stick to classic designs from classic brands and you won't have that problem. The basic white dress shirt/blouse for example works for all occasions and if you find a fit that works for you from an established brand you'll never have to worry about fit again (until you change size).