...brushless motors seem to be very hungry for voltage. seems like the more you give them the faster they eat it...
Yes, it's called Ohms law (Voltage = Current * Resistance)
Think of it this way:
Current translates to torque and the more you give it the higher the torque. Because super conductive motor windings aren't yet a thing you get hear loses. So basically current is limited to how hot your motor can get.
Voltage determins how much current can be pushed trough the motor, but also determins how fast the motor can spin, so you can run really high voltage if you can limit the current.
Electrical power is a product of Voltage * Current. A good motor at it's sweet spot can convert over 80% of that power to mechanical power (Torque * Rotational Speed), that gets used up by the propeller to lift your aircraft. Also spinning propellers very fast isn't very efficient.
...i was thinking of a low voltage relay that you could hook multiple batteries to to shed down one at a time...
Or you could get a couple of identical fresh (same wear level and abuse history) batteries and wire them in parallel including the balance leads, so if one cell from one pack discharges a little bit faster the other takes over more, keeping them both at the same voltage.
There's also a catch about the battery size: Bigger battery weighs more, so motors will need to draw more current to lift it, which drains it faster and also moves the motor away from its peek efficiency sweet spot, thus possibly shortening your flight times.
...now i just tried out some one shot 125 for the first time and i cant figure it out. maybe my rx doesnt support it idk...
OneShot125 needs a compatible flight controller (all LP stuff supports OneShot125), Rx has nothing to do with it unless you are trying to combine a PWM receiver, CC3D controller and OneShot ESCs, in which case it's a limitation of the flight controller not the Rx.