Power density hasn't really gone up in end user devices. 60W.h batteries are still 50% larger and heavier than 40W.h ones.
As for the rest, I've been trying to explain that while performance per watt has been increasing on AMD APUs, this has translated into more performance at the same power, not equal performance at lower power. Looking at their current roadmap, the same is to be expected within the next 2 or more years, with Strix Point.
This is how AMD has been tweaking their ideal voltage/clock curves. Below and above those curves their APUs lose power/performance efficiency, and that's why the ROG Ally (15-30W APU) even loses in performance against the Steam Deck (5-15W APU) when both are running at 10W TDP, despite the former using significantly more advanced tech (4nm vs 7nm, Zen4 vs Zen2, etc). Their current deal with Samsung probably prevents them from making an APU optimized for ULV (5W and below).
This is as direct and simple as I'm able to explain why your 6h battery life handheld fantasy isn't coming to fruition anytime soon. I'm sorry that I can't pass a message that you can understand.