Sometimes I get games a bit cheaper on Humble Store or Fanatical, but I wonder: is there's a way to see if it's even cheaper somewhere else?
Yes! There are price comparison websites, for example gg.deals. It sorts prices by best deal and tells you where to buy cheap Steam games. Note that it's not always a Steam key that you're buying, depends on the store. It also tells you if the game's available in some subsription based library or if it's even cheaper in keyshops, which you can turn off if you don't trust them. It also tracks the price history so you're always sure you're getting the best deal possible.