Hi @Maik_Fischer,
Welcome to the community form.
i agree, the setting and options for Dual Sim users are a bit limited on the Fairphone.
I dit had problems using dual sim on my Fairphone in the beginning that solved for me now.
i dont use dual SIM the way as you do, I do not have to switch my simcard off and i dont think my solution will work for you.
Maybe my workaround will help you to find a workaround that fits your situation.
wat i do:
it is possible to set a contract to a specific simcard.
only now after i have switcht one of my simcard off to see what you meant all my contact settings are gone as well 
… …that will not be useful for you to use.
Maybe there are ways to mute or redirect incoming calls from one simcard, i have not found that setting yet, that can be a solution for you.
maybe the setting is available on your provider’s website.
i think its useful to contact seport and explain the issue, and hope the setting will be improved feather in the future.
to contact Fairphone suport i noticed its best to use the Fairphone app, do not write again if you already have a open suport case.
Explain as clear as possible wat the problem is that you experience on your Fairphone. keep easy to understand and fast to read as possible to avoid unnecessary pressure on the currently overloaded customer support.
share so much information as possible to avoid the neet for them to ask for more information wat is time consuming for the employees.
Greetings
Tom