Thanks Andrew for your advice. I've managed to somewhat reduced the core brightness using HDR Wavelet without starmask. Some folks in Youtube suggested starmask in PixInsight when using HDRWavelet to protect the stars, I tried for many hours, yet to understand exactly whats I got to do. I also added in another 5x20min of Ha data.
Link to reprocessed Lagoon:
http://www.astrobin.com/full/49347/?mod=none