Hi @Isabelle,
I will change this post into a conversation as we're interested to hear about other users experiences and recommendations. I have found another post from our DATCH Community talking about this topic and I thought you might be interested to visit it:
Empfehlungen Steuerberater für Abrechnungen.
Also, I was able to find the following article with a list of partner I hope it is helpful for you:
Choose the right partner for your needs.
Please let me know if you have further questions or feel free to talk with other users in the thread I provided to you 
I hope you can find the right payroll office for your organisation! 
Wishing you a lovely Friday! 
Best,
Zulema