For starters, I'm thinking that the design will be more streamlined so that I can fit a 5Ah 21700 cell:

Secondly, I've been looking around for options that could reduce the workload, such as perhaps finding some off-the-shelf AliExpress smartwatch that has a custom firmware project active (there doesn't seem to be).

So far, the most promising thing I've found is this, the RK330Y Core Board:

It has support for RGB at 720p (I plan to output 1280x400 to an FPGA which will split to each display) and 8-channel audio output (I plan to try a 6-speaker, spacial audio headset implementation) through I2S. Unfortunately, I can't find any power consumption info and somewhat hoping that its use for IoT applications means that power consumption is relatively low.