Can confirm...I am sensitive the the iPhone X. Headaches, mild nausea, etc. I use my iPad all the time. No issues. As you said, the screen is IPS-LCD, not OLED. Rumors are that the upcoming iPads will not use OLED (I hope this is true since I’m planning to upgrade when new ones are released).
Also, I’ve been using an Apple Watch everyday since the day series 0 was released. Yes, it has an OLED screen, but it does not give my any issues. Could be as you said...smaller screen/less light. Could be a different OLED implementation. Could be just the fact that it’s not a device that is typically used for many minutes/hours at a time. Don’t know...but it doesn’t give me any problems like the X did.