How can this door tell me if it's locked or unlocked?

Hello Bammerman.
Thinking about the cosmetic comment, perhaps placing the magnet on the opposite side of the extension arm and paining the arm a less contrasting color would help. See the photo below.

SDBS 3M ed

Another way to sense the state of the lock would be to checkout the last picture of:
My Wyze Smart-ish Lock Another Approach II
The only thing seen in this application is the modified contact sensor sending unit placed against the wall.

Another approach is to install a Wyze Lock. It can be checked out at the Wyze website.

Victor Maletic.