Is it proper English to say:
"With method A, the goal is not so to perform task B but (rather) to address problem C."
Are there other more appropriate/elegant ways to convey the same meaning?
Use either "not so much" or "not", but not "not so".
With A, the goal is not so much X as it is Y.
With A, the goal is not X but Y.
The first means that the goal is more Y than X. The second means that the goal is not at all X; it is instead Y.