Skill actions tell you what their constraints and requirements are, so it shouldn’t be necessary to have knowledge of all feats. Just run them as written, and when a PC wants more than that, they can attempt it at a cost. This approach has the benefit of accommodating activities for which a feat hasn’t been created yet (since knowledge of all feats is unnecessary). As a rule of thumb, one should be safe increasing the DC, requiring more time, requiring more actions, etc because feats generally rely on having taken the feat as the cost for the benefit.
Sure, and I'm fine doing that but it creates more work than a better design.
Trained in Diplomacy
GM and players not studied feats
Player: "I'd like to Make an Impression on all 3 guards"
GM: "Sure, but increase the DC by 2 for multiple people at once"
Without knowledge of the feats, you don't have good signpoints on how to set the "cost".
what should I do? Is PF2 broken? has my responsibility of GM needing to memorize thousands of pages of text ruined the game? no, I answer thusly: "yeah, my bad. sorry, it was a call in the moment. the next time we need to swing across chandelier's you remind me that you have a feat for this, OK? thanks for keeping me honest ".
Agreed, It's not that big of a deal. Rational friends playing together (the only way to play IMO!) can move past it as you outlined. PF2E isn't broken by this, but I think it's an annoying design choice and doesn't add much for net negative. So I'm not as extreme as CapnZapp, but I definitely prefer other design options.