Its pretty simple, there are 3 Souls who does better than the others : Mordred, Andromeda and Siegfried.
Mordred is really good early game, but the main reason you're choosing her (debuff) loose its interest late game because a lot of kamihime does the same thing but better (for example, Belphegor has a more powerful version of her atk/def debuff)
Unlocking her isnt a loss though cause, as you said, Black Propaganda will be a powerful ex ability lategame, which will mitigate overdrive damage from the boss.
Andromeda is a soul everybody will unlock at some point for 2 reasons :
She is amazing, nothing much to tell. During raid everybody will benefit from her healing and she will rez you're kamihime (you can easily be os by a multihit). As long as she is alive, your group is alive.
Healer is a crucial role and you don't always have access to a good one (the luck dependent i was talking about)
Siegfried just hits like a truck on stun and will allow you to max your chance of getting MVP during raid, if played correctly. Also she will allow you to quickly deplete rage bar, when the boss is the most dangerous.
Whatever you choose between those 3, you wont be wrong.
About EX skill, it will be related to your actual team. Do you need more debuff? An attack buff? Maybe more hp regen or a damage skill? It might change every time you replace a kamihime. First thing you have to look at is "does this buff/debuff stacks with the one i already have?"
I will resume : each type of spells have 3 different frame (A, B and C). Same frame wont stack together.
For example : Dar't, Belphegor and Mordred's debuff (atk+def) are all B frame.
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Damn i wrote an entire novel again