I think these are both useful features. I personally like the idea of a shortcut to go back to the first unread post, and I think I wouldn’t have much use for “take me back to where I just was” if I had it.
“Take me back to where I just was” is very similar to features common in IDEs or in some ways can just be thought of as re-implementing the back button (because we want the back button to always jump you out of the topic?). I can see it being useful, but I’m not sure its much more useful than just the “take me to the first unread” post.
If one were much easier than the other to implement / maintain, I might just start there, and then see whether the next one really still feels necessary.
Or if both seem like distractions from the larger goal of a fancier scroll bar, maybe the effort is best focused there?