The effectivness of your magic phase is also very dependant on what lores you choose, and how many dice you use to cast each spell.
e.g. in your initial scenario, the enemy had two level two's, if he have an extra DD item, he will have 5 dice, and possibly three scrolls.
If you opt to cast spells on four dice, he will only be able to reliably dispell one spell with dice, and the chance to fail a spell on four dice is very low.
He will then have to use a scroll if he wants to dispell a second spell...
I personally prefer dark magic, and in some cases Metal over the other lores, Fire is generlly shiz IMO, just loads of S4 dmg spells, it's only ever usefull when facing nurgle demons...
Death is also usefull from time to time, Doom is especially powerfull if you have decent shooting.
Shadow have some good spells, but IMO, the first spell, and the crown are far to situational, I would only use shadow on a mage if I either have a lot of chars on foot, or if I have many mages.
Also, once again, I see mages as suport units, the only units you aim to whipe with magic, are other suport units, fast cav, skirmishers etc, otherwishe the aim is to cripple enemy units, so that your army can destroy them easier.
e.g. a unit of crossbowmen and shades will become very potent if you cast dome and darkness on a unit (panic with -3 LD), a unit of spears can easily dispatch an enemy knight unit if you get of a chare with march + lurker, a unit of spears can easily hold uo an enemy elite block if you sofhten them up with souls stealer or blach horror etc.
i hoped with this new book magic heavy is an option, but i guess it's more useful for one or two medium sorcs...
I'd concider 4-6 levels magic heavy, where 1-2 is the standard caddy.
Magic heavy is however perfectly viable IMO, the issue that I feel that you are making is that you have to many mages and to little army, e.g. the inclusion of 1-3 units of dark riders as suport for your army would always be usfull, where 10 units would be far to excessive.
Also are you hunting enemy mages enough? you should be able to get at least one mage down by turn 2-3 (depending on who goes first), IMO you should always include at least two units of harpeis for this, and preferably a unit of dark riders or two, don't be afraid of suicide charging the mages.
in my gaming group the people have fast armies
Fast armies means cav, cav are vurnable to RBT's, if you face a lot of cav, it would be a good idea to include two RBT's, place them far of from your main army, so if he goes to take them out (you can also guard them with harpies if he has light stuff himself, if he gets within 20 of the RBT's with fast cav, the harpies kill them etc.) he can't move down on your main force quickly.
Also, do your oponents use a lot of small units or many large? If it is the later, you can get a few more Dark rider / crossbowmen /harpies units, and deploy them first, this way, you can deploy your expencive units after he have deployed the most of his army, and this way get some distance, it is also very important that you march blook him early with shades/harpies etc.
It is also very possible that our different opinions about magic comes down from armylsit composition, e.g. I used one level four with focus familiar and a level two with the dagger, vs a demon army fielding four master mages, three pink horor units, and a lot of furies, that's a total of 9 DD and a lot of mage hunting units (all his mages were riding flying chariots, so were hard to pin down), yet, my mages did an excelent job, yes, I only get one or two spells through a turn, but since it was magic, I could get it where it was needed, which won me the battle.
I belive that the best idea if you want to go into a deeper discussion about how to get magic to work would be to write down an army list, e.g. the one fielding all those mages, and the oponent that you faced, then we can discus what lores to use, where to cast the spells, and how your mages interact with the rest of your list.