Let me teach you how to fish :)
So, I can't tell you exactly which small boutique consultancies to apply for. I can, however, provide you with the ways in which you can determine this for yourself!
- Network - I highly recommend you do some searches on LinkedIn for consultancies in Germany. From here, reach out to people at those very consultancies that look interesting or have a similar background to you (i.e. same university, from the same region in Germany, studied the same thing in university, etc.etc.). Ask them for some time so that you can better understand the company and what its like. From here, you should get plenty of phone calls and advice from all these people not just about the company they work for but where else you can look :)
- Search - Google is your friend here! Simply lookup "Best boutique consultancies in Germany". Scour google, reddit, glassdoor, etc. do get lists/rankings. What do you then do? Well, you can apply step #1 to these companies as well :)
In essence...talk to people! Ask for informational calls/coffee chats. Be very honest + clear in what you're looking for, and ask the pointed (and open-ended) questions that will help you better understand what each company is like!
Hope this helps!