The letter was a real letter, written about real people, going through a real situation, with real legal consequences. Your allegory works, because we’re all called to imitate Christ, and that’s part of the point Paul was making: Christianity isn’t an empty set of gestures and abstract beliefs, but permeates our entire worldview and colors the way we perceive and interact with our fellow humans.
So, someone can owe me $x. I can either evict them, because they haven’t paid rent, or I can say, “Hey, I’ll work with you.” And they can throw me a fraction of $x, just enough to act like they’re cooperating, but they ultimately take advantage of the mercy I show them and pay in their own sweet time, whatever they feel like they can spare, but I’m not really their priority, and keeping me happy isn’t their priority, and they don’t care one bit about honoring their obligations. So I can write an allegory about that as well— I’m like the patient and merciful God, and I know they’re flawed, just like the Israelites (or any other group of humanity), and I gently remind them, “Hey, you owe me a thousand dollars–” and they halfheartedly give me $200 or whatever, but don’t really make any effort to prioritize “paying the rent” over “eating at Hooters” or “buying new electronics for the kids”, just like we might throw a halfhearted prayer God’s way once in a while when we think about it, but we don’t really realize how much in debt we are to God for all his graces and generosity. And eventually, he’ll understandably grow tired of putting up with us, and he’ll allow the Babylonians to conquer us, just like I’ll eventually evict them when I’m ready to renovate the house they’ve trashed— and then they’ll be surprised, just like the Israelites were, and just like the Israelites said, “Why are you so mean, dear God? We love you! Why have you abandoned us?! We’re so sorry!!! We really do want to be your people again, for realz!!!” my ex-tenants will say, “You’re such a hard person, Midori!! We’re so sorry! We’ll be good, really!”
So, there’s an allegory about real people, with a real situation, and real legal consequences— we can look for parallels everywhere. But ultimately, it’s easiest to read things in the way they’re intended to be written: poetry as poetry, law as law, history as history, allegory as allegory, letters as letters. That’s not to say we can’t find any symbolism, or extrapolate allegory to apply certain truths into our own lives— but there’s plenty going on in all of Paul’s letters to be able to apply them into our lives, because he’s writing about truths that don’t just apply to Philemon and Onesimus, but apply to each of us in our own circumstances.