How to collect and manage bugs for a merge request?

We currently track bugs (alongside features) in an outside system (Asana) but are thinking of moving this to GitLab. We use merge request for code review already but are trying to work out how to get QA reporting bugs.

Bugs on master is easy enough: new issue, add steps and screenshots, done.

I’m trying to work out what the pattern is with merge requests. Ideally I’m looking to be able to create issues as children of the merge request, much like code review comments work. All of these issues should be resolved or spun out as project issues before the MR is merged.

I use the to-do list feature for small notes I need to deal with, but a single link of text cannot cope with a whole bug report with screenshots &c.

The closest I’ve found is the tester writes a comment on the MR, which does give some separation but besides a single checkbox there is no system of assigning it back to the tester after the issue is fixed. Plus these comments get lost rather easily in the discussion.

In Asana we can create subtasks that can have their own discussion thread, be assigned to people and do not pollute the project as a whole. How do I get that in GitLab?