Hello Stephanefrombrussel,
It sounds like you want to sync your Voice Memos in iCloud but not have them use up storage on your iPhone. We understand how important this is to you and we are here to help.
Currently when you sync with iCloud, they will be available on all devices but they will still use up storage on your iPhone. If you delete them from your iPhone, it will delete them form iCloud. More information an be found here: See your recordings on all your devices
We would also recommend leaving feedback for us here: iPhone Feedback
Cheers!