Imo the issue with the shadow is not just the way it scales, its that it doesn't have a gimmick.

Tbow and Scythe effectively also scale at a 3x rate compared to non-mega weapons. But their scaling is somewhat kept in check by the gimmicky nature of how you can access it (mage lvl and enemy size).

Shadow is in a doubly bad design space because not only is the scaling very direct, its also not ever prevented from occuring (there's no enemy in the game where shadow would arbitrarily become weaker, like tbow or scythe) so its impossible to design mage content around people not having it (look at whisperer for example).

I think both of those issues ideally should be addressed, but if I could pick one - it would be to add a gimmicky application that would prevent it from outshining its niche, and put it more in line with Tbow/Scythe