It will depend somewhat on what game creation/programming experiece you already have. Based on the fact you're asking about engine, I'd assume little to none. In that case: Just choose any one. You probably can't go wrong with Unity, since it's popularity means there are a lot of helpfull tutorials/guides out there. (Which then also makes AI better able to help you, answer any questions you have.)
Apart from that your idea sounds very ambitious, which is not bad, but you should proably learn the engine with a few smaller project first.
I agree with everything you said here ,Well your two main options are ...
Unreal will give you the best visuals, and you could probably build nearly all of it with their blueprints system and not have to code much if any. Unreal will likely need a stonger PC to Dev with. Its pretty complex and can definitely be overwhelming.
Unity is likely the easiest and quickest to learn, literally 1000s of videos tuts out there.. Unity has also been making up some ground recently in visuals. Keep in mind atleast with Unity is that it's a bit of a blackbox as far as their telemetry and spying on you.(Which they opening admit they do). We just dont dont know how closely they do ...
Godot is a popular open source alternative, although i have very little experience with it to say for sure, ive heard it cant handle big open worlds very well as it not optimized for 3D spaces.
Flax Engine is a good alternative to Unity as They both use C# scripting and scripts can typically be convert pretty easy back and forth as the Engine APIs are very similar. Its what i would consider a lite version of Unity, its new and still in development and has alot of missing features, but is full source code.
With that said if you dont have XP in building games you need to start small and make a fun little mini game of something.(Maybe one small aspect of your total game) There are 20+ aspects you need to learn, audio, animations, coding, shaders, 3D modeling, lighting, Scene optimization, etc .. it will likely take a few years to get good enough to build a open world game people would want to play.