I have a simple giro account at ING and just would like to query the balance regularly with a script. Nothing fancy at all. Is there some API you offer? I've seen your website https://developer.ing.com/ but it seems that developer access is only intended for developing activities and not intended for "productive" usage, even if it is with the giro bank account of the developer.
As far as I know it is currently difficult to get API access as an individual. The APIs are available to third party developers as is required by European regulation, but they have to go through a procedure to receive access to production data.
We have several teams working on improving our open APIs, including availability. If you reach out to me by email I will ask one of the teams and let you know when we expect to be able to offer what you're looking for. You can find my email in my profile.
Do not hesitate to post this question/request as a suggestion on the website, https://developer.ing.com/openbanking/support
You can check if your bank is supported here: https://myponto.com/en/reach
Check the API docs here: https://documentation.myponto.com/api