The German freelance visa is surprisingly easy to acquire. The only 2 things that are semi-difficult are registering a residence and showing you have German companies/individuals to work with.
For the residence permit, you just need the apartment/house owner to sign something saying you are living here as of this date. Then you take that note to the local Buergeramt and they register you as a resident. In some cities, I understand they will begin requiring a signed rental contract, but I just asked hosts on AirBNB before booking if they were willing to give me a note and finally found one who was willing to help (in Berlin). In the past, you could even just have a German friend write something that says you are living with them and it would work, though I’m not sure if they have tightened up on this.
For proof of work, you need 2 ‘Letters of Intent’ to hire from either an individual or a company in Germany that says they intend to hire you for freelance work at X hours per week and X euros per hour. You don’t actually have to work with these people afterward, it is not a contract, only a statement that says they Intend to work with you. Not that I’m advocating it, but this part is actually pretty easy to get a friend to type up and sign, especially if they have a small company of their own.
After those 2 things, everything else is easy and just busy-work. Insurance, bank statement, assets, your financial plan, etc. You will not need a business plan for the self-employed freelancer visa, just a page that shows your expected incoming and outgoing monthly income.
I went through this whole process a few months ago and received a 2-year visa, so if you have any more questions, feel free to ask.
Edit: I should note that this went smoothly because there is an economic need for people in IT.