That sounds like a good plan to me.
I think your assessment is right -- standard bilge
pumps, even large ones, will not do the job of dewatering the boat
in a flooding emergency
My solution is a very large trash pump with folding fire hose, which runs off 230v AC power. But I have a heavy duty generator
above the water
line to run it with. In the absence of that kind of power source, then I think propane is actually the best solution, because of the risk of carb gumming on a petrol one (plus difficulty of storing it), and because diesel
ones are just too big.
I generally hate propane -- it's inherently dangerous on a boat -- but if you have it on board anyway, this is, in my opinion, a really appropriate use case for it.
Besides trash pump you should, of course, have maintenance
pumps and volume pumps in each compartment, but I think you know that.
Don't forget a bilge alarm
, without which pumps might prove useless (once the sole boards start to float, it may be too late).