I'm failing to understand your guys' points. Did you read my entire idea?
Nothing is being stored on the imageboard servers, that's the entire point of it being self-hosted.
Some imageboards DO have logins, which I dislike for the exact reasons you guys mention (anonymity).
The service would be something you self-host, then it would be added as an option to "log in" on clients such as KurobaEx, or browser extensions such as 4chanX. This would of course require a pull request be submitted, then accepted. They can also be forked but the devs seem like they'd be open to such. If not, a fork of the project with this feature added is always possible too.
Anyways, the server itself needs to come before any of the client side stuff. The hardest part will be getting clients to add in the login feature, although it should be reasonably simple, some devs can be annoying yk.
But anyways back on point, after the user logs in, it would sync down all their watched threads, (You)s, etc. and it will be synced between each client/device they login to.
Everything is stored on the server, the server which is the program that I am suggesting be made. That program will run on a server that the user self-hosts, meaning it's run entirely by them, they have full control. The program would ideally also be as minimal, lightweight as possible. And of course free, libre and open source.
Nothing is being touched with the imageboards, nothing is being stored on imageboard servers besides your actual public posts to them as usual, the login system is completely self-hosted, and the login prompts are added to the end clients themselves.
I don't know how I can be anymore clearer than that. But you guys obviously didn't understand the point.