Both of these issues (dim incandescent bulbs and LEDs flickering/buzzing at higher brightness levels) are likely due to the buggy trailing edge waveform that I just posted about here:
You could try switching back to leading edge dimming as a temporary work around - I’ve been doing this by setting the switch type to “3-Way Dumb Switch”. This works even without having anything connected to the traveler terminal, though the switch will now produce relay click noises when turning on or off. I’m not sure if there are any other downsides to doing this. Do make sure that the traveler terminal on your switch is actually disconnected, though, as this mode will cause mains voltage to be present on the terminal.