I was able to port over several of my demo projects quickly because I originally wrote C# console applications and then refactored as I went when bringing it into a Unity project. I find this approach is actually easier for structure, as it decouples my code from the engine except for places where it’s absolutely necessary. I didn’t build it like this with the intention of moving over, but it made more sense to write my own logic for my own objects rather than make a game with a shit load of the bloated GameObjects Unity offers.
Swapping over to Godot was a breeze, all I had to do was learn the Godot specific calls for mostly the same stuff and plug and play. Super easy