I came across this posted on Reddit for the OnePlus 5, but I've tested it and it works on the 5t on OB8, at least. It's a Magisk module which activates always on ambient display to a schedule. You access the settings under Settings>Display
Thread is here:
https://www.reddit.com/r/oneplus/co...ble_always_on_display_for_op5?sort=confidence
Thread is here:
https://www.reddit.com/r/oneplus/co...ble_always_on_display_for_op5?sort=confidence