Hi @wonka1234 , If you want to export content to excel, SharePoint already has that feature.
From App, you need to have content first available locally in collection and then export to excel. You can try with little different action in flow. Rather then sending the collection content from app, you just need to send the SharePoint List name from app to Flow and use Get Items action in flow. Rest other things will remain same.
NOTE: Get Items action from SharePoint has default limit that it will give 100 items and if you want more then you need to change the limit from 100 items to 5000 items. Click on setting and change as per below. Also, you might have to use odata query filter for the performance of your flow.

I hope this helps.
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Thanks,
ANB