Q: easier way to slide the little time marker on the time bar

This is only for iTunes on the Mac...  I looked for possible keyboard commands for "incremental" forward or backward within the current track, and did not find any.  My suggestion is to make the progress bar as wide as possible, to make it easier to make smaller adjustments.  You can do this by making the iTunes as wide as possible on your screen.

But here's another way, using what I call the "secret" iTunes album art player.  While playing a song, along the top of iTunes window, find the ultra-small album art square at the left side of the Now Playing box.  Hold down the Command key and click this album art square.  It expands into a separate window that shows the album art, with large playback controls along bottom of window.  This is NOT the iTunes MiniPlayer.  You can manually enlarge this window, to be as large as your display allows.  Therefore, the progress bar at the bottom also gets wider and easier to click precisely.

I guess I just like clicking on random stuff, especially in complex apps like iTunes.    In an older version of iTunes, the album artwork was shown (much larger than current tiny square) at the bottom of the sidebar.  In that version, a (normal) click on it to expanded it into the separate album art player window.  So, I thought it might still be there, but it now requires a Command-Click.

This particular feature is pretty cool, so I don't know why Apple is "hiding" it so carefully.  I enjoy blowing up the album art to be as large (vertically) as my screen.  I often use iTunes in full screen mode, and the album art player even appears as a separate window on that screen.