@lister-of-smeg said in Red Light Power Flickering:

I would start by checking the rest of the peripherals (you mentioned having another pi - use that with the exact existing setup) That should let you know if the original board is faulty or not

If the board is randomly shutting down I suspect it will need replacing

Alright, I'll try that later today. Yeah I also figured it needed replacing.

Just thinking of how to prevent a repeat, could a split-second power outage have caused the Pi to die? While I was playing on it weeks ago, there was a quick power outage a week ago during a thunderstorm, not long enough for the PI to lose power or immediately cause this problem, but maybe that could have fried the electronics? Maybe I need to use a UPS or regulator

The page linked hints the power draw for the Sandisk to be below 3.5W, so it's not that much. If the disk draws 900mA during peak consumption, there's not much left to 1,200mA for the other 3 devices.
You can test by connecting all devices at once and performing some write intensive tasks on the SSD to see if anything disconnects. If that doesn't work, then get a powered USB3 hub.