BSM brightness?

I find the blind spot indicator in the mirrors fairly inconspicuous, especially in bright daylight. I didnt see a brightness adjustment in settings. Is there one?

No

This doesn't seem to apply in the exact situation you describe, but FYI, there is the capability to brighten the BSM indicators in certain situations when they're being dimmed by the automatic light control:

Function for cancelling illumination dimmer
If the BSM warning indicator lights are difficult to see due to glare from surrounding brightness when traveling on snow-covered roads or under foggy conditions, press the dimmer cancellation button to cancel the dimmer and increase the brightness of BSM warning indicator lights when they turn on.

blind spot indicator by the HUD or by the side view mirrors?

Really like the HUD BSM.