I got different answers from the people I talked to, the lawyer, accountant and insurance broker all told me different things, i.e. if I was to start a GmbH, a Gewerbe or be Selbständig/Freiberufler. In the end I went the Selbständig-route and it has been good so far.

Also since I have been 100% working for US based companies remotely I needed to figure out tax implications and there most of the time I had to research things own and explain to my accountant due to the the accountant not knowing English. I guess I could find a better accountant but now after a year the accountant knows how things work and I'm too lazy to switch now.

Other than that getting the insurances in place and bank accounts took some time, but not too bad in the end. But a guide would have helped. :)