I've never used the Kindle app, so I don't know what that user experience is like, but I've been pretty happy with Sumatra PDF for reading EPUBs. Inside the advanced settings you can adjust color, font, and font size. For me, that's all I need from an ebook reader.

Does Sumatra support reading Amazon's books direct, or do you need to pass them through DeDRM first? Or is there some other store selling DRM-free epubs I'm not aware of?

I end up stripping DRM off mine with Calibre's tools, then reading them on calibre-web.

Pretty sure Amazon's books are not directly supported.

Honestly, use Calibre and DeDRM. The Calibre has pretty nice epub/mobi/azw3 reading software as well as the best library management.

Or just get an actual Kindle.