In my case it was a desktop watch so logically it was powered on 24/7.
OLED displays are not like CRT. CRT tubes only decay when there is a long exposure to the front phosphor so as long as you do display changes (like screen saver) screen will be fine.
OLED displays does not work like CRT. Each OLED pixel has a short life span when powered, so doing 'screen savers' will not work as they will still power on the pixels. Will last a bit longer, but the OLED display is condemned since is power on.
The only good solution would be as you said, is having the display off, push a button, display info for 10 ~ 30s, then power it off again.
MongoDB is not really a lightweight database server in the sense the Omega is lightweight. For example if i recall correctly MongoDB starts off with 100 MByte initial size for an empty database. It is really more intended to run on servers than on embedded systems.
Maybe Redis woudl be a better choice (not sure)? Or what about somethin embedded like SQLite of BerkleyDB? Or maybe using Micropython in conjunction with their BTree module?