I cannot find an easy way to ONLY execute a step if it is the Matrix Aggregate. I was hoping "Never" on the section but then "Always" on the (checked) "Condition for Matrix Aggregation" would do it.

I think "Condition for Matrix Aggregation" should have a new "Only" option that only runs at the end on the parent run.

Currently, the workaround is "Execute Shell" and then:

if [ -z "${AXIS}" ]; then echo Master job, running next job; exit 0; fi
echo Not master job, aborting
exit 1