I would like to play. I don't mind which side.
I think you could draw up a list of a few spells (at least) from some school that the other side can use. You can ban that school from pretenders but many nations will have a mix on their mages even if mostly good/neutral or Evil.
Even if you look at death some of its spells are anti undead and so 'good' - e.g. Wither Bones & Dust to Dust. Plus the Evil ones are likely to throw a lot of undead at the Good guys and they may need these spells.
Astral could be used by both, but have a list of prohibated spells for the good guys - no Enslaves, Soul Slay (& therefore Mind Hunt), Soul Drain, Unravelling, Doom, Horror Mark, Abominations etc. Still leaving the Good guys with Wish, Golem, some combats, most astral magic items and the globals. Like Dispel
Fire should possibly be allowed for both? It fits well with Blood in Domminions 3 and is destructive (so evil?) but also has a clensing purity and fits well with the other Elemental magics for good? Maybe Water as well? That would still leave Earth, Nature and Air as solely with the good/neutral side?
If you think the Evil side will be too strong then you could restrict the cooperation on the Evil side. Maybe they cannot trade with each other in any way while the good side can? That would be quite thematic as the evil ones are united against the goods but don't actually trust each other.
But the Tartarians at least should be far less of a threat than normal. The Chalice it good so couldn't be made by Evil(?) As is Gift of Health and possibly Gift of Reason as they are both Nature spells. So there will be far fewer decent SC Tartarians than usual and lots more meat shields.