Is it a silly idea to have a UI where you open an accordion and another accordion appears inside, and when you tap it, multiple forms appear?
The goal is to hide as much as possible the fields where input is optional.
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If you dont have more than 5 - 6 input fields inside sub-accordion (if its height doesnt takes most of the screen) I would say this is OK.
It would make more sense though to place Icon on the left hand side before the text, as then this UI pattern then will be similiar to "Tree structure" which is very common and most Users understand it.
What you describe is called nested accordions and there are many examples on the internet. The problem is not nesting accordions, all depends on the type of design and its functionality.
Example demo at https://tympanus.net/Blueprints/NestedAccordion/