I have also only just started playing with a Mage at Impossible difficulty (for exactly the same reasons you've listed in the first post on the thread).
Based on my own prior experience of Mage on Hard, I think the key is going to be SACRIFICE.
I know based on some of the other threads on sacrifice that you're not a big fan of the spell, but take it from another Mage-Hard player, it is an awesome cost saver early-mid game and lets you keep your favourite units w/o excess travel.
Now, some people have used ancient vampires + sacrifice + letting them regain numbers, but I am not a fan of mixing undead with my troops.
The stategy that I used early and mid game was to get cyclops or giants, monitor your sacrifice strength to be just under the health of 1 cyclops/giant (if you have peacefulness, thats even better - you can increase health by 40% at its level 1 before casting sacrifice). Just cast the spell before close of battle - basically ensuring that you don't lose the cyclops/giant to enemy attack
Final point on the trick is if you have Orc Shamans (probably the most strategic unit in the game) hopefully in the Orc Embassy at Freedom Isles and giants, you can actually heal the giants enough using the magic axes for another go at the sacrifice spell. This is a great alternative to maxing out the healing spell (I prefer to not waste a spell turn on heal). It'll only work with giants though as Cyclops cannot be healed that way..
Btw, I will be looking out at this thread for swapping strategy being in the same boat as you
.