Supervisor wants me to support a diploma-thesis software tool after I graduated
I think you are being generous and, given what you say, should be able to step away from the project with a clear conscience. 

If he needs the software he should be willing to find a way to pay for support, maybe not you, I think, but some support person. 

I would see him if possible or send an email giving much of the information you've given here. The fact that you are doing very different things now and that support is disruptive to your main goals should be definitive. 

But I agree that it is good to find a way to keep a good relationship with him if you can also achieve your main goal.