Hello to everyone encountering this issue. After looking a lot at this problem I found out that a decimal percentage for a discount code can be set using the Shopify API:
First you need to create a Price Rule: https://shopify.dev/docs/admin-api/rest/reference/discounts/pricerule here it is possible to set a decimal number as the percentage.
And then associate a discount Code with it: https://shopify.dev/docs/admin-api/rest/reference/discounts/discountcode
NOTE: In the Shopify Admin the Correct percentage will be displayed in the Discount Code list only. In the single view it will be rounded!! I checked it and the calculation at checkout works correctly with the decimal discount.
It is annoying having to go through the API as it is really an advanced topic but I hope it helps!