DAX-Calculation best practice

There would be too much to expand upon. Grab a good book on DAX and study. THe best by far is "The Definitive Guide to DAX" by Alberto Ferrari and Marco Russo.

About your conclusions:

• Are you referring to creating flags that define Workgroup without GDPR? True or false, 1 or 0 so to speak. 
  • Yes. You should not perform free-form text searches in columns as this is slow (especially on large data sets). Instead, massage your data into the right format using Power Query.
• Not sure what you mean. What is it exactly I am doing that is filtering the table and how are you suggestion I should do?
  • This is something that you'll learn as soon as you start studying DAX properly. Read the book I mention. Basically, you should never place a full table as a filter in CALCULATE. Instead, you should only use the relevant columns with or without KEEPFILTERS depending on what you want to achieve.
• Never used SWITCH, so it's a preferable solution instead of nested IF-statements?
  • Yes, use SWITCH with many conditions as it's easier to read and maintain.