Oled screen allows only the pixels required to be lit up. So a 50% brightness thin line clock on Oled androids is pretty common.

Not months worth of battery saving like an eInk screen maybe but still enough to be insignificant compared to general "screen off" battery consumption.

Plenty of Android phones already do this. The Pixel 4a 5G is an example of one, and the always on clock / information screen drains very little battery, while still being really nice.

It is baffling to me that Apple still hasn't implemented this feature even on their top end "Pro" iPhones... but this feature by itself isn't enough for me to switch to Android full time.

The feature is not implemented on iPhones so that you are more incentivized to buy their smartwatch.

seeing as the phone is a chonker, you could maybe fit a thin screen on the top side of the phone, which would let you see some basic info without having to take the phone out of your pocket. (as long as the headphone jack was at the bottom and the power button on the side)