There is a difference between customs and immigration. Customs cares about things (and import duties on those things), Immigration cares about people.

As a US citizen, you should be able to just enter as long as your paperwork is in order. (And if Immigration is interested in you beyond the formalities, it means you are probably on some sort of list, and good luck to you!)

Customs will care mainly that you are only bringing in personal goods, and not commercial goods you intend to sell. They will also care if you buy any goods abroad to take back, because you may owe import duties. So they will not care about your Fairphone unless you bought it while in your trip abroad). In normal times you will have an exemption that allows you to bring in some amount of goods duty-free the the world is all topsy turvy now so I would check what the duty-free allowance is before coming back.

They are used to people bringing in phones, laptops, camera equipment, and other personal effects. They won’t cate unless you have a lot of things, and they think you are not being truthful when you say none of it was purchased abroad.