Awesome, would love to see it.
For the design – I think this will take some more planning, but I think it would be much better to take an "inline" approach. For example – the board title should probably be edited in place, like a normal Note/Todo. For columns, imagine a board is created by default with 2 columns - Backlog and Done. The user could click a gear icon (or pencil or something) and, in a popup, change to a tag or switch it to a "special" Backlog or Done column. Same thing for the Filters – that could be an icon on the board that let you filter the notes. By default it could be all notes in the notebook you added the board to.
Then, an advanced user can just switch to the YAML view if they want more control.
Let me know what you think. I am happy to sketch detailed designs of what I mean if you move forward with working on this.
The yaml looks good, I think one main difference if you are using the order of the columns in the list to define their placement instead of explicit numbers, that seems better than mine.
I see in the filters you allow for completed and subNotebook. I think that is great, although since it is a path, shouldn't that be "notebook"? or even "notebookPath"?
This might be expanding scope, but that made me realize some people would probably like to add a search as their filter. Maybe the work here: joplin-plugin-note-overview/README.md at master · JackGruber/joplin-plugin-note-overview · GitHub could help?
I'm guessing in the backlog, the "tag: backlog" is optional? Seems like that might get confusing, might be better to leave that line out completely. If the user doesn't want the "special" backlog, they can create a different normal column.
For the completed column, it looks like you went with the idea of a special column that marks a todo as done. That works well for my use case, but what will happen for "Notes"? In reality I think the best bet for a person using Notes could create a new column with tag "Done", so maybe that could be titled "todoCompleted" or something to signal that adding it to that column with switch to a todo and mark as done? Not sure the best solution for that.
All in all, good luck! I really hope that this gets made