I think I would go with a real pipe thread sealant on the threaded fittings, like a Loctite or Permatex brand sealant.
I'd probably go with a bedding compound on the thruhull rather than 5200. In the event there is a frozen seacock valve that needs to be unscrewed from the thruhull, I prefer to pull the thruhull and rebed it just in case wrenching on the valve damages the thruhull to hull seal. Maybe 5200 would hold the thruhull in place well enough that the seal won't be broken, but then again, maybe not. With 5200 though, I expect the thruhull would be a challenge to remove and rebed.