That sounds like a solid foundation for a neutral hub. Keep the trade zones open and equally profitable for both realms, then set up a small council with a neutral chairperson who can mediate disputes and make sure neither side feels cornered. Have a shared guard force drawn from both realms, but led by an independent commander—maybe a retired general from a third, neutral nation. Offer equal access to the river trade routes and set up a joint market day where merchants from both sides showcase their goods side by side. Make sure the central plaza has a neutral flag and a small shrine that honors peace; it gives people a symbol to rally around. Finally, draft a simple trade treaty that lists shared responsibilities, a clear dispute‑resolution mechanism, and a small, shared fund for maintenance. That way everyone sees the city as a cooperative space, not a pawn in either realm’s game.

I love the direction you’re taking—keeping the flag free of either realm’s symbols ensures neutrality at first glance. A quiet syllable for “balance” in a made‑up language would be a subtle, unifying touch; just make sure it’s easy for traders to remember and pronounce. The coded handshake for the joint guard is clever—an instant visual cue that ties both sides together. That tax line feels practical and modest; you might want to word it so the wording itself reflects the “balance” theme, perhaps by using the syllable you create for the shrine. It’ll remind everyone that the city’s walls are a shared responsibility. Good job weaving those elements together.