I agree Unreal and Unity are not appropriate but I do wonder about Godot. Its early enough where it doesn't have the strong connotations of being a game engine yet. I've seen some cool applications made in it too (
https://www.youtube.com/watch?v=9kKp0oguzr8). So I wonder if you could apply your energy to making it more cross platform using WASM (if that's even necessary) and extend it with your own UI language instead of rolling your own?