I was always trained to give speed instructions well in advance on TOD to ensure, like you say, that your descent can be planned accordingly, and for the most part I do this (usually only unable to if traffic is bunched and all aircraft arrive at or past TOD at the same time, which happens annoyingly often).
Why revert to explicit control "way too late" when all you have to do is tell each aircraft in succession to 'plan to reach WPT at TIMEZ and xxxKIAS'? Then they can figure out how to do that.
I can't understand why there is so much manual manipulation of descent and arrival profiles when there are numerous STARs that have all that speed and altitude profile stuff built in. All that's needed is a time at the beginning of the STAR, and the rest should sort itself out. If that's not working, then have somebody rewrite the STAR so it does what you need it to do! While there may be occasional unplanned conflicts, the STAR should be a 90% solution...