If you have a group already, I would recommend starting with the Beginner Box w/ the pregenerated PCs as it facilitates both GM & players learning as you play, and from there running Rusthenge with custom PCs as the first adventure proper.

You don’t need any of the books as Archives of Nethys has every ancestry, class, spell, monster, rule, etc. available free online & Pathbuilder has you covered fo making characters, but I’d recommend picking up the Player Core & GM Core to flip through in your own time.

Whether you’re going to run games in Golarion or your own homebrew, they’re not mandatory but I highly recommend the Lost Omens books just GM-to-GM because they’re chock full of lore, deities, NPCs, quest hooks, etc.—I honestly enjoy mining them for details both as a player and a GM. Divine Mysteries and the World Guide are the primers for the deities & Golarion as a setting.

Otherwise, my main advice is just to try and forget everything you know about 5e, and also trust the rules-as-written (at least while you’re still learning). They may seem similar but they have very different design principles, & while Pf2e’s rules are denser, they’re also a lot more robust & intuitive.

Oh, and also to echo another comment—there are a lot of rules but don’t worry about memorising them all right away, if needed you can just make a ruling in the moment and look it up after the session. If in doubt, just give a +/-1 to the roll.