Is there a publicly-accessible changelog for Mojave beta?
Short answer, no.

Apple doesn't provide release notes for public beta software. However, a comprehensive release notes report is available to registered developers enrolled in paid developer program (I am a paid registered developer).

As you mentioned, you are not looking for release notes from a developers perspective, but just need the broad strokes on what has changed. May I suggest looking through the one of the various blogs covering Apple news to get an idea.

One such popular site is MacRumors. They are generally quite prompt with any updates in the Apple ecosystem. MacRumors also share a birds eye view on the changelog with more user centric release notes.

See their blog post announcing macOS Mojave 10th public beta which outlines the user facing changes introduced in the macOS Mojave and notes the changes introduced since the last beta. Each post generally also link to the post for previous release and can be traced back to the first beta.