Agreed on the Xʀ being amazing, and about what you said about the 7+: a friend’s 7 on iOS 10 had the same battery life as my 6s on iOS 9. That same device on iOS 11 was getting 60% when compared to the exact same 6s on iOS 9. iOS 12 made it drop even further, I think it was 45-50% of my 6s on iOS 9.
I tested an iPod Touch 5G on iOS 6.0 in 2012: 6 hours and a half of 20% brightness web browsing.
I tested the same iPod Touch on iOS 6.0 in 2015: 6 hours and a half of 20% brightness web browsing.
I tested the same iPod Touch on iOS 6.0 in 2019: 6 hours and a half of 20% brightness web browsing.
Three tests, nearly 7 years apart, same iOS version = same result.
I couldn’t believe it. That’s why my 6s on iOS 9 was getting the exact same battery life with 87% health and three years later.
As you say, I’m also assuming that iOS 13 will absolutely demolish the SE and the 6s’s battery life. The plus might escape due to sheer battery size, that’s why iPads maintain good battery life when updated, even if they drop a little, it doesn’t really affect people too much.