Word on the street is that the castle isn't in the best of condition, which makes sense given it was built on a budget in the late 70s. As much as I'm nostalgic for it, I think it's ok to let it go.
The room the castle and Flume station (and south of the Flume station too) is pretty big, but not absurdly so. It's also an awkward space. Even so, one thing I liked about that area as a kid, and something I like in general, is rides that go through each other. Skyway's station was built in the middle of the Flume, which itself went over the 3D theatre and under Bermuda. Was rather interconnected which for my taste made things more interesting. I'd like if they could tightly squeeze a couple of coasters in that area, including going through storm; something high end thrill and smaller like a mouse.
I think the argument that SW is a family park and shouldn't have high end thrill rides is pretty silly. The thing about parks specialising and only doing one thing well is pretty new all things considered, and I think treating SW as a park only for families with small kids has been a big misstep. In the old days, these parks were for everyone, which means stuff the family can do together, but also a few proper thrill rides as well. If you balance out the attraction mix at SW with some more thrill rides, you would massively improve attendance. Don't neglect the families with small kids obviously, but don't ignore the older kids/adults too.
FWIW though, Surfrider and Vortex are both not good fits for SW. The only rides at SW with 140cm height restrictions should be one or two super headliners. In the context of a dry park like SW, both those rides are filler. Adding them limits what your next rides can be, while also not gaining you anything in terms of drawcards. In the context of a park like WnW however, both those rides would be 'big' thrill rides and a point of difference between WnW, and all the other water parks out there. Not only should surfrider not go to SW, Vortex should go to WnW. Vortex should then be replaced with a super low impact, quiet, family ride to give an aerial view of the dolphin pools/ski lake. Observation Tower, floating Island or Ferris wheel, take your pick of which it should be.