Thank you Stefan, I have sent you my address via PM.
Ian, thanks as well for your hint; I have tried eaDocX but it raised a lot of fatal exceptions which could not be resolved in the support because they could not be reproduced. So I have to admit that I kind of lost my trust in how nicely eaDocX and Word play together in our specific and maybe somewhat complex setups (no offense).
[edit]
After receiving the RTF template examples, here is my problem in more detail.
First of all, I created a custom list and some overrides. The manual instructs me to "select the List override radio button and type or select the list override item you have just created."
This is not possible. The dropdown menu is deactivated.
The output with a sample template is well-structured with sub-levels like "1.4.1.1":

Output with my template is flat:

It also uses Arial even though I had defined another font for body text.
The manual advises me to "Repeat steps 1 to 4 for the next level (Element)". But I do not want to number elements.
I want to number the sub packages without having to copy all the template contents inside the "child" section for each level:
child packages > < child packages.[/edit]