I know there are already many chorder apps but mostly to complicated, also nothing with AUV3 support and not really suited for use with external keyboards.
Something like the chorder in Presonus Studio One where you select a key note and than manually the chord notes you want to trigger, maybe including a fit to scale function and a save function to save your chord sets.
Important is that you can map a chord set where several keys trigger specific chords and the the rest of the keys play normal single notes.
Extra functions could be: Transpose,Key range setting for each chord and selecting a Midi Channel for IN and OUT
I have attached a screenshot
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