Did not test the alternator, but it appears to be in good working order. No lights, and with all accessories turned on, there is no apparent change. The battery appears to have plenty of strength. It turns the car quickly and never bogged despite repeated starting attempts. Once it is running, the car stays running fine when the battery is disconnected.

New info: I can disconnect battery, swap relays, and the car will run fine all day. It will not start the next morning however. I have to repeat the previous step.

Could this be a fuel filter issue? Seems kinda unrelated at this point but maybe?