Doing the modelling entirely in-engine with either Godot or Unity would be... challenging? You really need to be using blender or equivalent software for anything more complex than a box lol.
Unity has its own terrain editor and tree creator and while I believe there are 3rd party terrain plugins in the works for Godot, in both instances I'd still rather use blender (though I never got fully fluent with the unity systems).
Unity will have Waaay more tutorials and resources online so is probably easier to get into (maybe? Though I find Godot much easier to actually use!) but Godot isn't lacking in documentation and being open-source vs the money-grubbers that own Unity is a big plus for me lol
Shaders are always a little complex, I'd say creating them in either Unity or Godot is much the same process. The basic NTSC and film grain filters I use I originally found tutorials for online and then modified the parts that didn't work for me. Other than that I just make sure to only use simply geometry and set the game to a low resolution.
Good luck on your project! :D