So I did a bunch of research and landed on the only sane setup for a Xbox handheld that can be made by 2022/2023. Series S is will take another 3-4 years of mobile tech evolution to be fit into a sub 30W handheld.
The key motivation behind the device is hardware BC with Xbox One and dual use with a Surface device. Ironically MS’s misstep with Xbox Ones memory setup allows them to make a Xbox One portable with tech available today. Whereas there is no financially sensible way currently for Sony to get 176GB/s in a portable needed for PS4 BC.
With that being said here is the specs for a Xbox portable with Xbox One S BC.
Octocore Zen at 1.8+ghz. SMT disabled for XB1 BC.
6 RDNA2+ WGP(7 total) for 12 CUs at 1.3+ghz. ~2 teraflops. 16-32 ROPs.
32MB SLC (System level cache). Emulates Esram in BC mode. Only added if necessary for BC.
10GB of LPDDR5 6400mhz, 104GB/s. (2GB for OS).
512GB eMMC / SSD solution with lowest TDP. 1-2GB/s read.
1080p OLED. 8-9" screen. VRR.
4g/5g slot to enable Xcloud on the go.
The performance overhead should allow BC games with fluctuating framerates and resolution to hit their ceiling. The HW feature parity with Series consoles allows for easy porting from S/X builds. The performance target for these ports will be 30fps, perhaps unstable 60fps, instead of 60fps, and further reduction in resolution and settings from the S preset. Built in VRR screen will greatly help the performance targets.
The main sell of the device is a built for XCloud device that can play 99% of your existing digital library natively. I think by 2022, and even 2023 that should hold true.
Further clarification for dual use.
The same APU will be used in a Surface device with SMT on, bumped specs, etc.
I.e. 8 core Zen at 3ghz+ with SMT
14CUs at 2ghz. 3.5+ teraflops
16GB 6400 LPDDR5, 104.2 GB/s.
1 terabyte NVME SSD, 2+ GB/s.
30-50 W TDP.
Of course the Surface device will be sold as a gaming focused surface. It will be comparable to a Series S in perf.