I've used two methods to do this. One is stacking the hole saws, one inside to guide and one the size hole you want (can be the same as existing). This is a little messy but it works. The other method is to make a rudimentary puller, say a piece of metal with blocking on the outside of the hull bridging the fitting and some all thread running though to the inside. Apply pressure extracting the fitting, then walk away for a couple of hours. Come back and tighten the nut, walk away again. 5200 is very tough, but it will creep. This method may take all day, or even two.
I've tried the multitool but if the fitting is well glued in, it can still be very hard work getting the pieces to come out.