the differences between the fleets have become highly marked (or, at least, they would if my Hazat fleet did not comprise 50% ships from other fleets

).
I thought that was the difference with Hazat fleets?
It certainly tends to be with the Kurgan Grimson-o-rama. I kind of see the Sheerkhur as an investment opportunity, like a fixed-rate bond. Buy two now, and in two game's time you can replace them with a dreadnought... :mrgreen:
Li Halan broadsides are scary but their range advantage is only really about 6-12" - or to translate, one turn of movement with a 'crossing the T' type affair. Yes, they can try and manouvre to hold the range open, but if they do they tend to discover that whilst their broadside armament is awesome, their turrets tend to suck.
The key thing is that one turn of fire is painful but
not lethal. Accept you'll take damage and deal with it.
The biggest risk with boarding is your own ships - empty a ship of your own troops and it invites counter-boarding by another ship, but don't send enough and they'll be outnumbered. Of course, that's rarely a problem for Hazat or Vuldrok ships (which are laden with them), Kurgan (whos ships are slightly cheaper), or dedicated boarding ships like galliots.
It's one of those cases where the (fairly cheap) ship upgrades like psychics and elite crews can make a real difference.