There are e-ink phones.

They don't run regular Android though, with Google Play, just the open source version with some Chinese marketplace. I have followed one (now dead) e-ink phone on indiegogo (forgot the name, sorry), and they wrote that Google does not let (black-and-white) e-ink smartphones pass Android certification, because they cannot correctly display the colors in their apps. So they will never run Google Play, and there will never be just e-ink phone with built-in regular Android and Google Play, sadly.

There is a hacky way to get Google Play on the hisense phones, but... ugh. That's too much hackiness for me.

Usually, battery issues with hardware like that are more about idle time and reaching deep sleep states than active usage time with screen on. The latter is quite reasonable on the Librem 5, the former could be improved.

An e-ink display would allow for an always on clock though, which would be pretty sweet.

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)

Hisense A7 or Yota 3 should be your choice then. Both e-ink phones. Yota with the twist that you have the choice to use e-ink or a normal screen by turning the phone.

I would love an eink based mobile. Can't find anywhere selling the ones you've named in my country though. Back to hoping they'll become more common.