My guess is you will be fine, we are with the same size tank. There's a loose rule about how low you can go on the tank, something a little less than half so all 125 gallons are not really available. Talk to the propane company.

Time how long it takes to raise the spa 10 degrees and figure out how much propane you used with the 4.5 gal/hour rate. You can apply that math based on the water temp since it will obviously take longer starting at 55 than 80. I doubt you will actually heat the spa 8 hours per week since it will start cycling on/off once temp is reached. It will run non-stop until you hit the desired temp but then only about 15-20min/hour to maintain it while you enjoy!