What are my best options? Should I try for a normal employment with low hours or maybe do some freelancing? Maybe fiverr or upwork? Maybe a temporary employment agency? Thankful for any experiences and recommendations.
Working as a freelancer is a decent idea as well, due to the autonomy you'll retain, but the amount of time you'll spend on the business aspects of it could be substantial.
* I would reach out to early-stage startups. Many early startups don't have the capacity/budget to add full-time employees, but may be willing to take on someone flexible on an hourly basis who knows how to code and can help with other things as well. Bonus of course if you can get warm intros. The key is most places aren't advertising these types of roles, it's your job to convince them that you would be a value add and a low risk hire.
* What are your papers and research about? Is there a way you can turn this into advising or consulting based on your expertise?
* I wrote about the part-time tech landscape here: https://blog.parttimetech.io/p/the-part-time-tech-landscape. An expert network could be good if your research or papers are of interest to companies for quick cash. Fiverr/upwork/toptal type gigs are possible, but they take huge cuts and tend to be smaller engagements so you have to keep getting more work.
* I know in Germany companies are required to allow part-time work if it doesn't pose significant harm to the business. 10-15 hours may be bordering on too few hours for continuity (I feel you should try to do 15-20 if possible), but you should reach out to many software companies in Germany and see if they'd be willing to take you on from the get go at reduced hours. If you look at https://news.ycombinator.com/item?id=33422129 you'll notice many of the German companies offer part-time.
* I've started a job board for part-time roles: parttimetech.io. It's a little sparse right now, but we're working on filling it up. Maybe something there of interest.
Hope this is helpful, good luck.