Sorry NoobsDeSroobs ; english is not my mothertongue and I always forget about “many or much” rule 
And all my posts could contain some mistakes. Feel free to report them so I’ll get more understandable.
Well. The 3rd person game I’m thinking about is a game I really want to do ; like something really personal.
I’ve been thinking it would be easy as few characters to model / animate. But as I was writing down the ideas for the world, the rooms and the puzzles ; I kinda feel dizzy about the amount of work it would require to be fun.
And as it’s kinda my heart project, I don’t want to fail it or abandon it halfway 
Also, thanks for the softwares you’re quoting as I don’t know a lot of them yet. I’m going to check’em out.
I think starting with city builder is a good choice as it can be updated with time. No need to deliver all the planned content in one release. It’s harder to imagine doing that with an adventure game btw.
I was more thinking about a post-apo Tropico like game where you could pick your very own ethics and choices with some outland danger like raiders / other survivors camps to deal with. I was thinking about making a long / very long experience with maybe quests but no “campaign” or scripted levels ; more like only one city for hours but very reflective to the player choices.
So no big city management like Sim city but more very little cities like Tropico with citizens full of stats / traits.
I think I’m going to try the city builder. Just as you say, to learn the engine, put content little by little and still have an early playable version.
And finally, I’d like to thank you for your support. Thanks for helping me to get through this choice. See ya around really soon 
PS : About textures, I know that’s PBR is standard but I’m not sure about how it works yet. I feel like a good PBR needs normal mapping and makes games more grey. I need to read more documentation about it to have a better opinion.