[adwn]

> You can just as well just register as a freelancer [...]

Beware: If you're registered as a "Freiberufler", you're not allowed to sell or re-sell any physical or virtual products, including any kind of SaaS (but you're allowed to have employees). If you want to do that, you have to register a Gewerbe. To put it simple, as a Freiberufler you can only sell your time or that of your employees (it doesn't matter whether you use fixed-price contracts or hourly rates), but you can't sell your software.