Cathartic Mage Dedication is something of an odd feat, in that it grants a spell repertoire and the Cast a Spell activity if you don't already have spell slots, but augments your existing spellcasting if you have it:
If you don’t already cast spells from spell slots, you learn
to cast spontaneous spells and gain the Cast a Spell activity.
You gain a spell repertoire with one cantrip of your choice,
from a spell list of your choice. […]If you can already cast spells from spell slots, you learn one
additional cantrip from your spellcasting tradition. If you’re
a prepared caster, you can prepare this spell in addition to
your usual cantrips per day; if you’re a spontaneous caster,
you add this cantrip to your spell repertoire.
So far, so good. But we run into a bit of a problem when we take Basic Cathartic Spellcasting, which grants the basic spellcasting benefits. Those benefits were written for archetypes that grant spellcasting rather than expanding it, and it's not entirely clear whether they should do anything.
So: If a character that can already cast spells from spell slots takes Cathartic Mage Dedication and Basic Cathartic Spellcasting, what happens to their spell slots? I'm especially interested in how they would interact with bounded spellcasting classes like the Summoner.
Best Answer
When the dedication feat does not provide the spellcasting feature, feats providing spellcasting benefits should provide those benefits to your class's spellcasting feature. That is to say, a fourth-level bard who has Basic Cathartic Spellcasting has a total of 4 1st-level and 3 2nd-level bard spell slots: three of each from being a fourth-level Bard and an additional 1st-level slot from Basic Cathartic Spellcasting.
Bounded spellcasters work the same, even though that does get a bit weird. A fifth-level summoner who has Basic Cathartic Spellcasting has a total of 1 1st-, 2 2nd-, and 2 3rd-level summoner spell slots: the 2nd- and 3rd-level slots from being a fifth-level summoner, and the 1st-level slot from Basic Cathartic Spellcasting.