There should be an option to grant Send As delegation to the Microsoft 365 group
Currently group owners can add and remove group members. However, if they need the members to be able to reply to emails using Send As, that delegation has to be set by an admin. On top of that, the delegation can only be assigned to individuals. So every time the group membership changes, the delegation rights are out of date. Instead, there should be a check box in the General settings area, that says "Allow group members to Send As the group".