You could add another extra step of customization by adding a ‘kit’ system like in old school (not action) rpgs of Baldurs Gate 2 and Pillars of Eternity 2.
Each kit would have some characterization in lore so different types of night blades or what have you. And like those games I mentioned a kit introduces a bonus and a penalty. Or more than one bonus and penalty. So you could choose ‘no kit’ and be the same we have now or you could choose one of a limited number of kits for each dual mastery.
Sadly it is not ideal because there are so many mastery combinations compared to character classes in those games eg Pillars of Eternity I think has ony 11 character classes. So you could go 2 different kits for each dual mastery + vanilla instead of 3 or more kits.