About the suggested milestone #2 we reached it in july/2017 (see here). It is an indicative that people are voting and it is good to have more people around who able to answer questions.
However, the proportion of unanswered questions is growing faster than the ones which do get answers. We agree a significant part of the former group are the type of poor quality, off-topic, uncler, broad, opinion-based and duplicates (some references: here and here).
If I would answer this question today, I'd say a milestone would be somehow to reduce the number of unanswered questions by being more effective towards questions which should be put on hold/closed/deleted. This could be achieved in many ways, such as going from improvements in the SE system bot which automatically deletes poor posts; having more 3k reputation users who are able to cast close/reopen votes; giving more incentive to users to find duplicates (for example); and also having current 3k users to be less tolerant with poor questions (including questions from new users) and quickly put those on hold, independent if a custom comment will be posted or not. It is time to rely more on our default close-reason messages (they can be improved if needed) plus the extended documentation built on CV Meta along past years; and put more weight on the OP's shoulders about the responsibility of learning how the site works and come back to improve their questions if they can.
1- One milestone I think we could be proud of:
- the respectful environment we have been keeping on Cross Validated SE;
Merits to all community, but specially for the high quality level we have on our moderation team;
2- Milestones yet to reach are difficult to say, but what about:
Mathematics SE has 190 10k reputation users (1068 questions by 10k user).
We have 22 10k reputation users (1318 questions by each 10k user).
I think this milestone is important because it brings us more independence as a group.
It is also a consequence of having many users participating and voting.