Gamma os is pretty well optimized at this point. Running an older build on the 405m and it’s extremely stable. Have not used a rg350m but did have a rg351mp and rg351m, the 405m is a noticeably thinner device. No issues with the screen or anything as that seems to be solved with later batches. Build quality and feel in the hands is phenomenal. Only complaint is the case does get warm on the side near the processor when running cpu under high load. Though you can open it and use some stacked thermal pads to make contact between the cpu shield and back case to even out the thermal distribution a bit.
The android build the 405m ships with performs worse than the rp3+/ flip as it uses a lower performance gpu driver. Gamma OS uses the gpu driver from the builds for the rp3+ and flip to bring performance back in line with those devices.
As an aside, huge benefit is that the 405m is a 4:3 device that has enough cpu to run Sega Genesis games using cycle accurate ym2612 emulation (massive sound quality upgrade). It’s a perfect companion device to a 16:9 handheld like the rp3+ or any of the Odin handhelds. At the moment cycle between an Odin pro, steam deck and 405m depending on the game.