This has been described here https://experienceleaguecommunities.adobe.com/t5/adobe-target-questions/how-to-set-target-cookie-wit... and I haven't found any solution so far. I have also raised ticket with Support Team.
This security issue is of high priority in the security audit report at one of the Target accounts I am supporting on and it is a blocker at present. Their security team wants to have all Adobe cookies with secure flag. It has been already set for other Adobe solutions but there is no result giving workaround for Target.
When targetGlobalSettings approach didn't work, I tried adding a custom code (which is checking document.cookie and on finding Target cookies it is setting secure flag with other information but the expiry/max age value of the cookies is getting messed up) in Launch before loading Target and we can't proceed with this implementation until we can confirm that there won't be any impact of this approach to Target's functionality.