Steam – How to turn all the cards into gems at once

Ah, I found my answer at the verrry bottom of the Steam Trading Cards FAQ:

Do I get Trading Cards for completing achievements in game?

Trading Card drops are currently just tied to playtime within a game, independent of which achievements you are getting. We will definitely look into adding more rules for how cards are dropped in the future, but there are a few problems with in-game achievements that prevented us from using them right now. Many games have achievements that are grindy or un-fun to get, and we want the act of getting game cards to be fun for all users in games. We also don't want to push users to cheat or use achievement-unlockers in games in order to get cards, since that devalues achievements and isn't that fun either.

Having just looked into this same challenge, Steam Inventory Helper (the originally-linked-to answer) has been reported for spying on its users. The replacement recommended in the linked Reddit discussion is Steam Economy Enhancer userscript. N.B. It must be installed through Tampermonkey (or Violentmonkey), but is open source and thus far, well received in the community.

It also works well on Firefox. Source: Personal experience of unloading all of my Steam cards in the last five minutes...