GIANT Bicycle Taillight NUMEN TL1 Mountain Bike LED Flash Warming Taillught Red - Lights & Reflectors - Bike Accessories

Maybe just stick with this. Pretty bright, visible from the side, good mount, they seem to claim 50 hr run time.

I want something that will be there when needed, & when the alkaline batteries get low the light dims rather than cutting out entirely.

Another guy on the same ride had (pretty sure) the Serfas light. It died too.