Accessing tags from the renderer is not currently possible neither from plugins nor in the app, and I'm not certain it will be. What would you like to implement specifically? Could you use a custom webview for this?
which will be visualized like trello/kanban board.
Each card in the board was supposed to have a button "Done" that would move it to the last column.
Also user was supposed to be able to drag'n'drop cards between columns.
I don't know the best way to implement this in Joplin. Maybe a "custom webview" would work as well, depends what do you mean.
That will give you much more control over your content. In comparison, the markdown-it plugins are mostly to render Markdown to HTML and not really designed for advanced interactivity. Maybe in your case, you can use a combination of both.
In current implementation of Joplin+plugins, can I possibly make the panel to take the whole space that would otherwise be used for the rendered markdown?