Possible workaround to the solution mark notification:
Set up a hidden board. Make a new topic and subscribe to it.
Replace the mark as solution button with a custom one that runs an endpoint.
Have the endpoint mark the solution, post a reply (with any custom text you need) to that topic in the hidden board, and reload the page, so the user sees their marked solution.
Now, anyone who subscribes to that topic will be notified.
It's definitely a workaround and not a permanent solution, but it's a flow that works for me on several custom components.