Old question and you’ve probably moved on by now and I don’t know if this will help anyone, but.
acf_form() can’t be used inside an already existing form like this. For this you’d need to use some other functions that are not in the documentation. For something like this I would suggest starting by looking at how acf_form renders the field group, follow the code. Then if you have specific questions about getting it to work start a new topic.
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