Oh, I get it. Good on you for looking to support American companies!
For an alternative, I'd suggest checking out
onyxbooxusa.com. It ticks most of your boxes: it's not a Kindle, has a 7-inch screen, offers a dark mode, and sideloading ebooks is straightforward. Plus, it boasts a flush screen design, which is a nice touch. The Onyx Boox series is known for its versatility and Android-based system, which means you can use a variety of reading apps and formats. Hope this helps!