Isn’t a milestone essentially a group of To-dos? I would like to be able to group To-dos into a milestone and when all to-dos are completed, the milestone is, too.

You can do this already. The only downside is the milestone needs to be created before you start setting up the To Do lists. When the To Do list is first created, or edited, there is an option to relate it to an existing milestone.

You can then select if the completion of a To Do list also completes the Milestone.

There is a “posting this message completes the milestone” for messages & comments linked to milestones, but I don’t see the same option for Todo Lists. It would be a very useful feature, though. As would showing the existinence / option of linking symmetrically (on both milestone & todo).