Good work! Thanks for testing with the PF cells. Did you happen to take any readings that showed the increasing output thing again?
Can you flash a STAR firmware build with a lower freq PWM? HarleyQuin posted a little chart with a writeup on what does what. I’m pretty sure that all you’d need to do to drop from 19kHz to 2.4kHz is set your low fuse to 0×65. CPU freq would not change, only the divider.