I'm also going to experiment with putting a thermal pad underneath the DC-DC module in order to enhance heat dissipation from the underside of the module to the PCB.
The goal here is twofold: better cooling of the module (where almost all of the board's heat is generated), but also better thermal coupling from the module to the I2C temperature sensor, enabling more accurate self monitoring.
If things go well this should be the final module design, and I can go make a bunch of these to use for powering all of my future large instrumentation / networking projects.