Which smartphone displays can you set to 'incredibly low brightness'? all iPhones and Androids I've used are like bright flashlights when the lights are switched off.

It's all relative. I don't own a single flashlight that can be set dimmer (or anywhere close to as dim) as my iPhone XS Max is at minimum brightness on a white page. If the XS Max is showing mostly-dark content, the amount of light emitted is really tiny at minimum brightness because of that OLED display.

On the various Android devices I've owned, I've used a useful app called Screen Filter that adds a translucent layer on top of all apps to dim the screen beyond the artificial manufacturer-imposed minimum brightness, which worked especially well on OLED Android phones, but also LCDs worked fine too.

Presumably, all or most Xiaomi phones. I've never used one for long, but the lowest backlight level in those I tested (Redmi Note 2, Mi A1, RN5) was comparable to the level 2 or 3 of the Kindle Paperwhite frontlight. (level 1 is a failsafe option since the Paperwhite has no physical buttons, it's not meant for reading, being too dark even for a pitch black room)

That said, I don't think reading in poor lighting conditions is a good idea for your eyes, no matter the technology.

You can use an app to reduce the brightness further. I've been using Twilight on Android, and it does indeed get much dimmer. You will certainly lose some image quality/performance, but it's very nice for viewing in a dark room.