Disc brakes feel spongy no matter how much I bleed them

also, if the brake pedal is not firm, sinks slowly to the floor with foot pressure,
Do a close inspection of each brake line fitting for a very small leak.
A very tiny leak, a very tiny drop is enough to cause the pressure to fall off. Wipe each fitting with a dry paper towel, pump the brake pedal, go back and re check the fitting with a dry towel. If nothing is found go back and do the same test on each hard and rubber line.