Would Godot be suitable? Godot officially supports C#.
https://docs.godotengine.org/en/stable/getting_started/introduction/introduction_to_godot.html#programming-languages
Godot is definetly a strong contender. I would have to look into it a bit more to confirm it has all the features that I require first, but the fact that it supports C# is a strong argumument in for its case. Thank you for for the suggestion!
