I have a crowd-sourced project going on to try to get an estimation on the skill acquisition rates based on players’ input: Driftsmelting Result Survey 漂移錬成結果調査
Based on the submissions so far, some skills appear to be harder to come by than other skills. However, the sample size is still small, so if you and your friends can help contribute all your future driftsmelt results, that will be helpful in making the results more generalisable and representative.
The project currently does not take in results for event-limited driftstones and drfitgems.