Sub tasks are just issues with a parent issue. When displaying them, always display the ancestor crumb trail.

Marking an issue as as a subtask of another. Having task lists in a description is a great start, but it doesn't help (AFAIK) navigating from child back up the chain to parent. Creating umbrella issues is a very common way to track the top-level focus areas for a release.

subtasks would give insight into how long it will take!

One of the reasons that some projects don't use Gitlab's issues and use an external tracking platform is the lack of issues relations. Without relations issues are just flat, no way to actually track progress of big features. No way to create a "meta" issue that depends on 4 other or create subtasks and so on. The same problem exists on Github too. It would surely make a difference if Gitlab offers a full features tracking issue, instead of just flat issues. Relations is a major first step towards that.