Hi,
I built an automation to sum up story points in an epic based on this question and the answer from Carlos Treminio.
Sum up Story points to epic
As discussed there, this automation doesn't work, when a story gets added or removed to/from an epic. The solution posted by Guy Anela doesn't work for me, because the mentioned smart value {{changelog.Epic Link}} gives nothing back, which I tested by logging it.
So I tried the following solution for adding a story to an epic, this works great: 
When adding a parent issue to a child issue, my idea is, to populate a custom field (customfield_10271) with the issue key of the parent issue, which I can use for the next automation, when I remove the parent issue from a child issue.
But I'm not quite there yet. When this automation runs, the story points of the epic is always 0, I don't understand, why.
The automation looks like this:

In the log of the smartvalue {{issue.key}} I get the correct issue key of the parent issue, which was removed. In the last step I try to build the sum with {{lookupIssues.Story Points.sum}}, which always is 0.
Thanks to everyone, who read my question up to this point and I'm happy about every input from the community.
Best regards
Jan