I don't know how true this is in the real game, but in the combat simulator, Ulm's biggest problem tends to be fatigue. The unit descriptions say they have "high endurance" but is seems to have no effect on their ability to fight in heavy armor
Giving all ulmish units +1 strength and +2 reinvigoration would be thematic and effective.
As for countering Ulm's magic weakness, here's a thematic possibility, the "Arcane Cripple" :
"More susceptable to magic than their brethren, some poor Ulmians have been twisted and scarred by its harsh energies even while yet unborn. While their presence reaffirms Ulm's long-held fear of magic power, Master Smiths sometimes harness their weakness to acquire the rare gems necessary in advanced forging, using a Cripple's pain like a lodestone to find sites of magic power."
Arcane Cripples are sacred, have rather low stats, are feebleminded, and have another random disability, and MR=6. They are leaders, with 0 leadership and no special actions. However, every turn they have a random chance of discovering a magic site in their province - any magic site, regardless of path or level. They would be quite cheap (maybe 30 gold, depending on the chance of finding a site each turn).
The other possiblity:
Master Jeweler.
"Though best known for forging tools of war, Ulm's metallurgical talents also encompass the area of fine jewelry. The reknowned Ulmish Master Jewelers will search far and wide for prized gemstones to endow upon their latest masterpiece."
Master Jewelers would also have no magic paths, but could use the special command "Look For Pretty Jewels" to find any gem-producing site up to level 1 in any path. Like a level-1 rainbow mage, but including holy/unholy, and unable to find sites like "Damned Merchant" which do not produce any gems. Master Jewelers should cost about 250g and get a random gem in their inventory every season (not every month) regardless of their activities.
Possible Ulm beefing units
Threaded Mode