Just a note: there are few RTW Mods actually doing something about rebellions, one of them, the most famous, is Roma Surrectum, where Big Rebellion (now toned down) is one of the most atrocious and glorious experiences in gaming history, then, there is Rome Total Realism, where you can find an interesting (cruel) feature which turns your most successful generals into traitors, but in this case, the traitor general isn't anymore under your control.
For a long time I've thought that using the loyalty system and the shadow factions of BI together with the Senate mechanics of RTW, it might have been be possible getting close to a simplified but interesting depiction of the Imperial Civil Wars. Now I'm almost certain that it's impossible, but, until somebody forbids dreamig, I'll go on with my dreams!
Returning to the most difficult aspect of playing: I think that too many enemies stacks, more than increasing the challenge, they increase the boredom, but I well know being alone on this subject.
Then about the difficulty of maintaining the army, I would agree that it was a very important aspect of Late Antiquity, but I think that this difficulty comes after the political crisis, not before it. I mean that the financial problems the Romans had to face during the III and IV centuries, came after the begin of the political crisis, i.e., without the political crisis of the Severian dinsaty (even though I think it's the Reign of Commodus the actual turning point), the Roman Emperors would have been able to maintain their armies as it happened before. So for me the Revolts are a more relevant factor, than the upkeep of the army.