I would strongly recommend you look into Obsidian (obsidian.md). You're definitely right about the GTD demographic being apple-centric, but this is a cross platform mind mapping application I've grown to love.
I was actually one of those incredibly young kids to finish school, and this right here scares me. Nothing good came from the rush other than some local publicity for a few months. I think it crippled my chances long term social skills, and still going to therapy to try and catch up in any non-academic sense.
Is this site saying that for 240 dollars I can buy LTSB? I've been trying to get a hold of a copy but unsure about the legality of those 30 dollar keys online since I don't understand licensing.