I think that all sounds fine. Since your tank is fairly large, I wonder, does it have a single inlet/outlet or does it have multiple? If you had multiple, it would be a good idea to slightly pressurize the tank with compressed air and vent forcefully to ensure you get any trapped pockets.
If you only have a single inlet, make sure to turn the tank so that the outlet is at the lowest point of the tank, since propane is denser than air. This also applies to carbon dioxide, which is slightly lighter than propane as well, so, trying to displace the propane with carbon dioxide will work much better if the outlet is at the bottom of the tank.
That being said, if you're filling it with water, you'd have displaced any propane anyway, the carbon dioxide would just serve to displace any oxygen in the tank to a level below the LEL. With all the precautions you're taking, I wouldn't even bother using the reciprocating saw, just use an angle grinder, it's so much easier, and I can't see any issues arising due to sparks or anything.