Simply create a feed and call it, say, bookmark.

If a user bookmark a post on, say another feed called "timeline", then you would add reaction on that post of kind called, say, "bookmark", and at the same time you would use "targetFeeds" to put it there.

Example: const bookmark = await client.reactions.add( 'bookmark', {id: '6d47e970-d317-11eb-8080-800109d57843'}, {}, { targetFeeds: ["bookmark:zoro"] } );

Then that feed will have these bookmarks.

You can check "own reaction" to see if activity on main feed (e.g timeline), which you did make the bookmark from, is already bookmarked or not and show banner to user or icon. And of course you have now the bookmark feed which can list all the bookmarks.

All likes/comments would be visible on all feeds, as this is fan out.