Yeah, and I think that kinda puts you in this tough spot. It’s my understanding that the display is really the biggest power draw on a laptop since it’s pretty much a continuous draw whereas the CPU can drop down to just a few watts for a large amount of the uptime, rarely drawing as much as the display does. That said, I can’t actually say for sure there is much power saved by choosing a 13.5" over a 16" panel. I can’t find good numbers anywhere so it too may be negligible, especially if you use your brightness wisely!
There are still all the additional components that the 16 has that are all opportunities for parasitic drains even if you don’t use them, so I’m still skeptical if a U-series CPU would really give you much. I’m just guessing about all this, though, and you’re totally right that it does fit the spirit of the design to have as many options as they can sell. If only laptops could return to socketed CPU’s!