I've had these for so long that I completely forgot they weren't default behavior of the site (lol oops). Main reason why I held off on sharing was I couldn't be bothered to make a userstyles.world account and learn the semantics for publishing stuff like this. I did now, finally! Under each heading below, I've included links to both userstyles.world and github where these styles can be grabbed from.
Full-width Feed Selector

Stretches the width of the "latest posts" / "bookmarked tags" button at the top to align with posts underneath it.
(two more styles below the 'read more': sticky avatars and sticky sidebar)
Sticky Avatars
Makes the avatars next to posts stay in view as you're scrolling like they would over on tumblr. That way you can remind yourself of whose post you're looking at without having to scroll up (unless it's a reshare, you're just gonna see the avatar of who shared it).
Responsive Sticky Sidebar
There are a few of these already and I'm not sure which one I originally adapted it off of. My version focuses on just the main menu bar and fixes some selector issues I experienced with the original. I also added a margin on the bottom so that it lines up with posts on the feed (might be off on some pages, but the "latest posts" feed is what's mainly used so I prioritized that).
Recently, I combined this userstyle with a couple others to compact the sidebar on smaller screen sizes and to add a scrollwheel while viewing bookmarked tags. Credit where it's due to @twilight-sparkle and and @binarycat for the parts they've written.

we try to click like on posts that we rebug to help our memory, sorry if that's annoying!