I think it would be very helpful to have an option to add notes to individual posts. Something like a yellow sticker, that I can simply click on to enter any text. This information should only be visible to myself and assist me to find previous posts of interests.
There should then be a possibility in my Content view, where I can list all my yellow stickers, for example:
Upon clicking the item, it will open the corresponding post.
There is a similar existing feature to add notes to bookmarks and make them private, but it's cumbersome and only applies to threads, which I find not as useful than adding my notes to individual posts. When making a bookmark private, the OP also receives a message like "... was added and removed" or similar.
Yellow stickers should always be private and not send any notifications, and again, apply to individual posts, not threads.