I understand the importance of being well educated when servicing the German market, although you seem to be doing okay so far bringing in business. However, as per Joe's answer, there is a sense in which being an entrepreneur is about being prepared to turn your back on the conventional job market, and standing or falling on your own skills and determination alone. If you're not up for that at some point, then maybe you're better off staying employed. Neither course is wrong - different things suit different people - but trying to mix the two is unlikely to work long-term.
A bit of practical input based on my own experience. Seeing as you have the luxury, choose your customers based on their ability to help your business develop. That doesn't necessarily mean going with the firms offering you the most money, but rather firms that have requirements in line with your long-term product strategy, or firms that bring such tremendous kudos as customers that the mention of their name makes your future sales so much easier.
Again picking up on Joe's point, also look for projects where you can use outside help and get the customer to pay for it, whilst building out the product at the same time. Be very careful about that one "big-name" customer who wants the earth, especially if much of what they want isn't strategic to you - they can kill your business with their constant demands (my previous business once had this experience with one of the 'big 4' consulting firms - not nice). Along this line, look for firms that understand where you are as a business - ones that you feel you can work well with and are prepared for a bit of give-and-take.
A final unrelated point, possibly superfluous as Germans are typically very efficient and organised - make sure you have your basic support systems in place (see for example the Joel Test) - it's probably obvious (although it wasn't to me 8 years ago) but a small business can drown in quality and support issues if it isn't properly organised.