It seem like this is not possible in TYPO3 13 anymore.

See: https://docs.typo3.org/c/typo3/cms-core/main/en-us/Changelog/13.0/Breaking-102970-NoDatabaseRelationsInFlexFormSectionContainers.html

It throws an error:

Invalid flex form data structure on field name "overlay" with element "settings.infoPoints" in section container "container_infoPoints": Nesting elements that have database relations in flex form sections is not allowed.

Has anyone fond a solution yet?