I migrated from an app called Tomboy forever ago to Evernote, then from Evernote to Google Keep and Google Dogs which sucked because it was completely manual. Then I converted from Keep and Docs to Obsidian a couple years ago. Moving from Keep to Obsidian was fairly easy because of Google Takeout, now made much easier with plugins. Converting from Docs was a somewhat manual process, but I've basically got it done. Anything remaining in Google Docs is now an actual document rather than notes.
My ancient Evernote content was re-imported this year using a plugin and put into an Archive folder. Since there were a lot of bookmarks stored in there, it's somewhat handy to have it in an archive folder because I'll see it in search results. Importing the images and whatnot was helpful too, because the initial move to Keep lost a ton of formatting and content alignment.
There is a community plugin named "Importer" that works pretty well, and you should take a look at it. To force the issue of cleanup, I like to have a property assigned to notes for cleaned up or summarized so I can use a dataview to search for notes missing that property or with the property set to false/empty. That way I can go back at my leisure to clean up, summarize, tag, link, etc.