I rode my bike across the country last summer so I badly need to replace my cogs / cassette. I have a Giant Defy 1 with

cranks FSA Gossamer MegaExo, 30/39/50
cassette Shimano 105 12/27, 10-speed

Are there any other parts I might want to replace (aside from the chain)? Do the teeth on the derailleur wear out at about the same time? Probably a stupid question, but I'm new to actually doing any work on my bike, lol

I assume you've measured the chain for stretch, or the cassette is rough, otherwise you might be replacing parts that still have decent life in them. Unles you're sure the cassette and chainrings are toast, take it one step at a time.

Measure the chain and decide if it's time to replace it (1/2-3/4% stretch or 1/16"+/12"). If so, replace the chain only and see how it runs. If it runs OK you can put off replacing the cassette and chainring. Otherwise try replacing the cassette but hold off on the chainrings which generally last much longer. With good care you should replace 3 or more chains before replacing the cassette.

Lastly if the chainrings are shot, decide if it's only one or all three, in which case you have an economic decision because often 3 chainrings cost more than an entire crankset.

As for the RD pulleys, most last a long time because they don't run under much load. (the ones on my old road bike are over 50,000 miles old and still fine). Examine them by eye and listen or feel for vibration as the chain runs on them, if you don't see, hear, feel a problem you don't have one.

Just replace the chain if the cogs are bad. Are there any other parts to replace after a cross country ride? Possibly. We can't inspect it here so take it to a shop for an evaluation of bearings etc.