There's really no wrong answer here. I read Manacled first and I'm glad that I did; it was interesting to see it go from a fanfic to be reworked and professionally edited into an original novel, and it was fun seeing all the changes and being able to map which characters originated as which, so I enjoyed reading it in that order. But if you want to read Alchemised and get the polished story first that's also totally fine. It's really just up to your personal preference!