Interesting. Definitely sounds like a connection issue. I hate to say it, but can you try restarting your device? Or can you try the same device on a different network (e.g., hotspot to your Android device?)
OK so I figured this out. I was a little suspicious of the “Login status” in the debug info being “logged in” which I don’t remember ever doing. When I uninstalled the Mac app it retains some info between installs, including this weird login status. There’s a note on where that data is stored. I just went into that directory and deleted the obsidian folder. When I reinstalled the app, it let me set up sync as expected.
It seems like my account got into some weird state that just prevented it from loading, and that state was preserved between installs by default.