As someone who went for Andromeda before Mordred I can tell, the upgrade from Cassiopeia to Andromeda is really not worth the 500 SP when you're getting near the rank 50's

I absolutely recommend getting Cassiopeia and after going for Mordred, the debuff might not be as good as D'artagnan's but it will work while you climb up the ladder of the extra 4rth skills as you unlock other souls with useful skills that can complement Mordred's

In a case scenario in which you don't have a good Weapon to he held by Mordred well, while you branch out into another elements try not wasting the Soul Points in Legendary souls but the ones before them