First, I can't go so far as to say this is a healthy relationship. Is it better than your past of dysfunctional relationships? Sure. But you're sort of ignoring the fact that this relationship is dysfunctional.

You've been together for six months and are already engaged. Why? Even if we don't sit here and make an argument that this is entirely rushed, look at the entire premise of this post; you don't feel like you're in love with your partner. If that's the case, why would you agree to enter into a legal relationship with him?

You list out some very important attributes as it relates to compatibility. That's great. But you can't sit here and say that you have no doubt you'd have a happy and secure future together when you're again, literally here questioning whether you're in love. That's not someone who's secure in their relationship.

I'm happy that you're at least holding off on wedding planning until you figure this out. To address the situation around the "spark," what I'm struggling with here is how you define it. To me (I'm a guy and I'm happily married for context), the "spark" is working together as a team to remind yourselves why you fell in love in the first place. You don't need to be doing new and exciting things 24/7, but it can't never be happening either.

You see the spark as an obsession. Obsession is unhealthy. You should never need someone; you should want someone. But there's a difference between just not being obsessed with someone compared to feeling generally apathetic about the person. It feels like the latter for you, and if that's the case, why would you be in a relationship with them, let alone be considering marriage?