Working on them day in day out, the 777 has the most space because it's the broadest cabin.
The 350 IS narrower than the 787. Sure the high ceilings and vertical sidewalls make the 350 seem spacious, but it's an illusion.
The 787 is only 2-3-2 in WT+ rather than 2-4-2 on the ironic* Airbus. This actually makes the cabin more special on the 787. Not to mention much easier to work in. Both are 3-3-3 in WT Vs 3-4-3 on the 777 and previously the 747.
On the 787 you can turn a single trolley between the toilets in Economy, on the A350 you risk skinning your knuckles just going up the aisle with a trolley. And don't get me started on the config. Alot of crew either actively avoid the 350 or suffer it for the destinations it serves.
*I say ironic because it's labelled Xtra Wide Body. It isn't. Unless you compare it to the A320 family as a point of reference. Flight crews dislike it too because Airbus tried to make it fly like an A330... Which it isn't so it doesn't.
The A350 is best described as just enough... And that's not a compliment. Given free choice I wouldn't operate on it ever again and do actively avoid it.
I understand where you're coming from re the windows on the 787, and if someone asked, I would gladly unlock a window to allow free viewing. That said, on a night flight back to Blighty, one window will illuminate the whole cabin when the sun rises... And coming back from say, Seattle, mid summer, it basically doesn't get dark, which is sub optimal for a night flight, hence dimming the windows or dropping the blinds.
Tother thing of course with the 787 is that the windows are about 50% bigger so more light etc.