Just to say I've tried this today and love it. Was running Authelia but wasn't happy with the way it was configured. I spun up VoidAuth in about 5mins and am impressed. Running as basic auth for a Caddy reverse proxy, this feels very slick. Well done!
Easy to follow documentation as well. Less is more sometimes.
I like the way you have it checking SMTP credentials in the background with a simple 'this is not configured properly' on the UI.