I am a bit confused why you think it is not easy. In fact you have the right to reduce your hours from full time to part time if your company employs more than 15 people. So you can easily make time for a freelancing job on the side.

Also you don't really need to track your hours when freelancing other than maybe for billing purposes so you really don't need to worry about hours anyway. Generally you are considered part-time self-employed when doing less than 18 hours per week.

Earning a bit on the side is really not an issue in Germany. In fact the combination of having a part time employed job and then doing freelancing is very popular.

What doesn't work is being full time employed at two companies but that would make no sense even if you could as you would earn much less and pay insane taxes.