I think either of the first two would be preferable, though personally I think the second option is the best one.
As it is currently the bookmark section is buried several menus deep (and I would almost say not intuitive at all on mobile). In most cases I would think having a direct link to a couple topics you want would be far more useful than say categories, because for the majority of sites (especially if trying to be efficient, which would be the group to utilize the sidebar at all) they don’t have such an expansive range of categories that having a one-click to ___ section is more convenient/saves times/whatever the use case. This is site specific of course, I’m sure some have lots of new topics in categories whereas some have a lot of posts in specific topics in the categories but even looking at it this way I’d say bookmarked/followed topics would be AT LEAST as useful and relevant/helpful as tags and categories. I have a mix of both on my site but my users all have a couple of (different) specific topics that they’re most interested in, and then will just browse the general categories.
There’s also already a precedent for having something in the sidebar and also in menus; both PM Inbox and Chat channels are (imo) more easily accessible from anywhere on any page in their normal spot in the top right then navigating to bookmarks which requires completely leaving the page into preferences.
All that aside, if that would be too cluttered Option #1 would serve a similarly useful function. The old Hamburger Menu had a theme selector plugin that was VASTLY easier than navigating multiple menus to change your theme (literally two clicks from anywhere) and I wouldn’t be surprised if a poll was taken a decent % of users wouldn’t even know they can change their theme from their profile unless they were specifically directed to it (then remembered) vs seeing it in the front menu (convenience aside) and IMO this applies even moreso to bookmarks. The Theme Selector has also now been reintroduced into the redesigned sidebar, it shows up at the bottom by the keyboard shortcut menu which makes it fantastically accessible and immediately noticeable.
If the sidebar wasn’t a thing I wouldn’t particularly have any feelings about changing it, but since it exists (and there’s comparatively a lot of space) it makes sense with the existing use of it. You could even make it so the sidebar sections themselves (tags/categories/bookmarks) can be turned on/off by users if crowding is a concern (you can already just leave them blank)