Minor victory is mine!
There were some dark times in the final decade - famine, disease, bankruptcy, and a lot of war - but, amazingly, diplomacy saved me in the end. I managed to seal peace treaties with the Picts and Varinians, who had been harassing my Gallic provinces for years. They promptly went south into Spain and distracted Hispania and Gaul, allowing me to retake Narbo and Tarraco, which I had lost to them earlier, and to move my armies back to defend my Alpine regions.
Peace treaties with the Quadians and Marcomans in the north east freed up more manpower to fight the Huns. Against them I fought one of the most epic battles I've had in my time playing Total War: three full stacks of quality units each. They had the superior units, I had a slight numerical advantage and the high ground. As you can only have 40 units on the field at one time there was a continuous stream of reinforcements coming in from both sides as units routed or died, constantly changing the face of the battle. 2500 of my men died, including my faction heir and greatest general, but I won. The Huns fled not long after and eventually requested a peace treaty.
Prosperity returned after that. By the end, the only known factions I was at war with were the Ebdanians, who were still freezing and starving in Britain, and Gaul, who had been reduced to a couple of provinces in south-eastern Spain. With everyone else there was peace, and even friendly relations. Finally, a bit of colonisation and military alliances with Macedonia (through subjugation), Illyria, and Eastern Roman Separatists (through diplomacy) got me the eastern provinces I needed to secure victory.
I might return to this campaign later and go for one of the higher victory objectives, but for now I think I'll go with the Huns next, using the Radious mod.