Do you mind telling me why you think this setup would be too cumbersome ?
You provide the user with a connect link/qr code the user presses this link/scans the qr code and gives you read only acces to one of his/hers accounts via the prompt in the bunq app. 🤔
This way only 1 api key is needed which is only yours.
Im just curios here and would like more details on your point of view.😄👍