Have you ever thought about learning until an advanced level? When yes, why? I can’t see a reason except it’s necessary to fulfil a requirement for a job application or similar. But for your daily conversation, an upper intermediate level is enough?
Using infrequent vocabulary is not helpful. The majority of people would have a problem understanding you. More useful is the usage of colloquial language. Before starting a language course for a level C1 or C2, think about your exact needs. Do you need it for daily conversation or in a medical or law environment? Focus on your needs rather than on a system with levels you never will use.