Grady the post timeline (“post scrubber”) can be hidden with the following CSS (ref.) (paste in Admin > Appearance > Custom CSS):
.DiscussionPage-nav > ul > li.item-scrubber {
display: none !important;
}
As for moving the “Reply” and “Follow” buttons, that’s trickier - the post content could be wider, and the buttons would cover it in that case. Or screw the rest of the layout. Also, extensions can add more buttons there, which complicates things further.
This is the best CSS that I could come up with:
.DiscussionPage .container {
display: flex !important;
flex-direction: column;
gap: 0;
}
.DiscussionPage .DiscussionPage-nav {
align-self: end;
width: 9rem;
}
It positions the post side navbar in top right corner and overlays it on top of the content as you scroll, e. g.:

When scrolling:
