This is how not to waste resource, fill it till 100%:
How long have you been playing? Were you around at January 2019?
This issue have been brought up in the forum and has been resolved.
Before January 2019, even after reaching 3/50, the probability to raise special skill is still low as before, 2% for same color 1*, 4% for same color 2*.
However, since January 2019, SG has addresed this issue and make improvement. Nowadays, at max level, same color 1* give 10% while same color 2* give 20%.
In short, SG have implemented improvement in January 2019, the improvement is: after reaching max ascension/level, the probability of raising special skill level is 5 times bigger than before.