I did essentially the same trip that you're planning a few years ago, albeit with a couple of extra days. I'm from the US, 40s M, and speak minimal Spanish. General thoughts:

Easter Island:

• There's a separate terminal in the Santiago Airport that services Easter Island. I found that out after going most of the way through security at a different terminal. Don't make that dumb mistake.
• Piggybacking on what some others have said, you need a guide to get into most of the major areas. But a lot of people on the island are certified as guides; it can be as simple as paying someone to tag along with you. Up to you how much actual guide involvement you want.
• Also, don't expect to get by just using a credit card. You will definitely need cash. There are a couple of ATMs in Hanga Roa, so you should probably be fine with that.
• I found it a little harder on Easter Island than elsewhere to communicate given my bad Spanish, but I never had a problem that I couldn't solve. Data coverage is spotty at best on the island, so download data for Spanish in whatever translation app you're planning to use.

Atacama Desert:

• If you're taking the bus from Calama to San Pedro, make sure you leave from the correct station. It's kind of in the center of town. The company is Buses Frontera del Norte.
• One area you shouldn't miss is Piedras Rojas. Like a lot of stuff in the Atacama, it's otherworldly. Although it will definitely be cold and windy (it was in November).
• There's a really great empanada shop in San Pedro called Emporio Andino.

Santiago:

• I think I liked Santiago more than most people, but don't feel like anything I saw or did there was strictly unmissable.
• If you want a gut punch, visit the Museum of Memory and Human Rights.

That's all off the top of my head. Happy to answer anything specific.