[Illuminated One]

Being able to fend of a rush isn't the only problem that the weak nations have.
You will also need spells to make your troops effective against normal troops (indies, PD), so you end up with a much smaller empire and not in the position to overrun anyone (while the rush nation will find someone unless you make the counters really overpowered).
And honestly playing 30 turns just researching counters while your strategy looks like "archers in a big blob fire whatever, infantry in three small blobs hold and attack" isn't very funny imo.
If you want to keep it as simple as possible just give weak nations x points they can spend on research however they like before the game starts via map modding.