Arco is precisely a fine example of "excellent mages, crapish troops". Mystics are a serious candidate for best non-cap mages in the whole game. But they're significantly held back by so-so national troops and no thug chassis either.
Even if they get no blood/astral/death, something like MA Ulm shows that simple E2/something 1 mages can be pretty brutal. Summon Earth power and then go to town with magma spells, Strength of Giants, Blade Wind, Marble Warriors, Rain of Stones, Earth Meld, Maws of the Earth, etc. Lack of Legions of Steel is hardly a problem. Lots of A2 mages spamming lighting wins games. Fire 2 goes to town with Phoenix power then fire elemental spam or plain blasting. W2 is for quickening and ice strike and fighting underwater. I'm hardly taking in account any crosspaths here.
And you don't need to make multiple nations at once, just pick your favorite element/city/gym and start there.
Or if you really insist in 20% of non-cap mages to get level 3 magic in one of 5 paths of your choice, then at least add the terrain limitations mentioned before (with the cap able to recruit any).
Forests-Nature.
Coast-Water.
Mountains-earth.
Wasteland-fire.
Plains-Air.
There, at least that way you can't just spam the right element to hard counter your current opponent.