The OP asks,
how to express healthy curiosity, unambiguously?
The OP assumes that curious, meaning a desire to learn, is neutral. I disagree. Curious (in this meaning) can be positive, neutral, mildly negative or very negative. Which will be conveyed by the context, if written or the tone, if spoken.
There is no single word, which by itself, will unambiguously express a healthy desire to learn. If written, the context must clarify what is meant by curious. If spoken, the tone and manner will clarify what is meant.
For example (written):
Jane wants to major in physics. She will succeed because she is good
at math, and, more importantly, because she is curious about
This is very different from writing:
Jane is curious; she never smiles.
"How curious you are!"
This statement can be approving or disapproving, depending on the tone. If spoken coldly with a disdainful expression on her face, the speaker is saying: "How nosy you are!" If said with a pleasant laugh it can mean "How odd you are!", and if said with an unpleasant laugh it can mean "How nosy you are!" If the speaker means it as a compliment, she will probably add something: "How curious you are! You are interested in everything!"
The OED says, of curious:
Desirous of seeing or knowing; eager to learn; inquisitive. Often with
condemnatory connotation: Desirous of knowing what one has no right to
know, or what does not concern one, prying. (The current subjective
The OED also has a definition of curious meaning odd, but the principal ambiguity in speech will be between healthy curiosity and nosiness.