This seems like a great idea that I strongly support. But as someone who went down this road, I have some tips as I’d love to see this succeed and it’s really easy to burn out trying to do this.
First, I would recommend breaking it into chunks. Don’t do more than a certain number (15? 20?) in a sitting. Most of them will be easy, but the tricky ones will be really tricky.
I would not try to do all of those tasks at the same time. Or if you do, expect to get relatively few done in a single sitting.
The grouping of duplicate issues is a particularly thorny problem as without a good structure to rely on, it can quickly become an n^2 problem. It would benefit you to have the tags & milestones in place before trying to group them into duplicates.
Closing issues that are believed to be fixed tends to require diving into the details of what’s being reported and it can be incredibly draining. The biggest problem for these activities are the common cases where an issue is originally reported as being about one topic, when it later evolves into a different topic as more folks weigh in.
Re-titling issues also faces the problem that the issue originally created may not be the issue as it evolves. However, in some cases, it’s not possible to have a single title that encompasses everything that is discussed. I never really figured out how to resolve that one… it’s by definition a bit intractable.
Milestones have less to do with the structure of issues and more to do with how the project organizes it’s progress. I think you should avoid trying to make those decisions as you go through this (even though it means doing a second pass later).
I hope the lessons I learned from earlier attempts end up helping this effort! This would be wonderful for the