Updated the FAQ with more info. This thing is complete as far as I am concerned, even though there would be more stuff to write about.
As for my personal progress, I am on day 63 now. I'll adjust the expected playtime for completion up to 350 hours from the initially estimated 300. List of achievements I got:
26. Made all kinds of badges. (Doesn't need to be all on the same character.)
27. Reached level 255. (Finally!)
28. All A-ranks in boss challenge mode. (Did this one with Link.)
29. Twilight map completed. (Not too shabby.)
30. All A-ranks on the Twilight map. (Wasn't easy.)
31. Earned all rewards on the Twilight map. (Worst map of the big four.)
32. Helped 10 Online-Links. (That took some time until I got around to do that.)
33. Completed 5 skulltulla pictures. (Only triggers if you completed the five pictures from the base game.)
34. Unlocked all 8-bit weapons. (They are rewards on the Master Quest, Twilight and Termina maps.)
35. Termina map completed. (Joy!)
36. All A-ranks on the Termina map. (Not as hard as the Twilight map.)
37. Earned all rewards on the Termina map. (That was the last one!)
While it's not tied to any achievements, I completed the Extra map in adventure mode as well; this means I am done with adventure mode. What's left to do is wrap up legends mode on hero difficulty, complete challenge mode with all characters with an A-rank, get all characters to level 255, forge an "ultimate" weapon for all 30 kinds in the game, and earn the remaining seven achievements. Not sure if I'll get all of that done in the next 37 days because in May and June there will be retail releases I am actually interested in.