I think this very much depends on how easy it is to get notes from your existing system into Markdown. I built a tool to export my notes from Onenote to markdown, which made it simple to get into Obsidian and VS Code (I use the Markdown Memo extension there, but am looking forward to the development of Foam). Evernote can export to Notion or Joplin, which can go to Markdown. Other apps have other workflows that have been shared in the forums. If you can do it easily, I think the answer is definitely “Bulk Import”, as the curation has to happen one way or another, and it might as well be in the place and format (e.g. wikilinks, tags, headers, etc…) that you want to be using.
The hard part, of course, is actually making sense of it all. I’ve been “curating” (if you can call my paralysis that) my 6 years and million words of “notes” for a couple months now.
I spent a while “refactoring” many monolithic monstrosities into more “atomic” notes, and now have thousands of them. I’m currently setting up some topical Index Pages a la the IMF system and will start cataloging these atomic notes into MOCs, where I’ll then aim to get them to a sort of “Evergreen” status eventually, probably deleting all the excess (while keeping an Archive of it for posterity’s sake).
If I started again, I think I would have set up the MOCs first as I could have been tagging the pages as I refactored them. Not only would I have saved time, but I would have had a better understanding of what goes where and how the MOC categories should be structured.
I hope this is helpful to someone…