Main is described as "The most supported devices, with all the features and stability you'd expect from a regular OS."

Unfortunately there was/is no device supported by postmarketOS that fits that description. You'll need at least good telephony support including 4G features like VoLTE, proper camera support (not potato polaroid from the 80s quality), Wifi, Bluetooth, geolocation, working GPU acceleration, media hardware decoders, decent battery life. And I'm probably forgetting a few things.

Let's hope that initiatives like https://liberux.net/ will help make a fully working, long lasting device available!