In my experience that is something you just tell your place of deployment beforehand. I never had such a contract, and many of my colleagues run small businesses next to work as well. That is Switzerland tho.
I communicated that I need maybe 1 hour per week for random issues from work time and that was ok.
I think what I am trying to say is that you can always talk to your employer. Reduce working hours to 80%, take less money and give it a try (if they allow it)
> Moreover, I'm considering leaving a very well paid software developer job to free myself of this obligation
I would also think about what we'll paid means for you. You can always get paid more if you agree to less free time and freedom. If youre pretty and young you could make bank with OF, but at what price? Is it actually a good job if it doesn't allow you what you want to do in your free time?