In terms of pure power efficiency something RISC-V based might be the way to go. Obviously, OS and application compatibility is an issue when you step outside the x86 and ARM ecosystems, but in that regard, as a Linux weirdo, I have a leg up, given that when a piece of hardware comes out, no matter how niche, someone out there with enough interest and skill will port Linux and it's many applications to said hardware and then it's out there in the aether because open source (try to beat that run-on sentence). Indeed, in a few years, when the battery on my current phone starts to die, I will consider a phone running RISC-V with probably a custom version of Linux. I'm not a power user with phones, I use it to make calls, send text messages, and watch an occasional video, that's pretty much it. I'm growing increasingly frustrated with Android and wouldn't mind distancing myself from "Do No Evil" Google, but I have no desire to have anything to do with Apple's walled garden either.
Speaking of efficient, and getting back to AMD, 7040U Phoenix ultrathin laptops and handhelds should be hitting the market any day. I've seen some dubious benchmarks out there that look really promising, but nothing reliable yet. Being able to run pretty much anything at 1080p at ~20W is the dream. 🤞