It's a combination of me wanting me to mod the likes of Civilization IV: Colonization and Doom 3 but getting frustrated that I was hitting limitations with what those games allowed and not getting custom sounds to work in Doom 3 but in UE4. But yeah, I would like to have money too. One of my goals would be to sell my own version of Civ4Col where the Kings are more involved than just financially exploiting their colonies and only use their Royal Expeditionary Forces to pummel their colonies if they try to rebel. Seeing as the European powers did militarily assist and defend their colonies in real life.
So basically you want to make a spiritual sequel of Civ 4: Colonization? That's probably doable as long as you're either willing to outsource the models or become a really good 3D artist. From a programming perspective, it shouldn't be very hard. In this specific instance, either engine should suit you fine. Games like Civ don't require amazing graphics or advanced stuff like raytracing. In fact, I'd argue excessive lighting would detract from that sort of game.
Sprites are a pain in the ass. I've made custom monsters and weapons for Doom 2 and know this. Models are tricky too, but I haven't invested the time necessary into learning how animating works to really complain about that yet. If I can get models I make to use the animations I download or the ones that came with the engine, then it's just a matter of making the skeleton and naming everything the same as the ThirdPersonCharacter.
I would highly recommend not using default assets except for testing purposes. Nothing makes a game feel cheaper. Even if you download free assets from the internet, that's still a much better route. Animations aren't
as noticeable as models, but someone will notice and meme on your game and then you're "that guy who uses Unreal Engine default animations".
I've heard the argument you're making applied to Quake 3 Arena maps and Battlefront maps. People told me to make basic and IMO boring generic Quake 3 maps, instead of following my ideas. While I kind of understand now because such levels are made to accommodate for the fact that AI bot generation isn't something you have much control over, trying to make a level with complex features helped me learn how to use them, and how to use them well. I feel it's best I try to work on what I'm motivated to work on, otherwise it feels like a boring slog rather than something I'm pumped to work through. Is this a false equivalence?
It depends on your level of experience. When I first started making games, I had no idea how complicated something as simple as an inventory system or dialog box could be. You really do have to start at something incredibly simple and work your way up. Mapping isn't quite the same, because mapping ultimately isn't a mechanically complex thing. It's more of an art than a science. Video game logic is an absolute science and if you don't do it exactly correct, everything will break and you'll often have no idea why.
If you have experience with programming, especially OOP, then sure, start making a Civ clone. But do it iteratively. Make a game where you can move a settler around an empty map. Then implement cities. Then implement resources. And so forth. Don't try to build everything at the same time, because having 40 different partially-implemented systems will overwhelm you. It would overwhelm anyone. It's better to fully implement one system, then add on another.
In the game I'm making I'm using other people's free assets as placeholders until I learn how to make my own models that function. I have experience using Autodesk Softimage 7.5 when modelling for Star Wars Battlefront, but that mod tool I'm not allowed to make any money off of models I make with it. So now I am trying to learn how to use Blender.
Free assets are fine. Asset flips make tons of money all the time, as long as they're fun. Don't spread yourself too thin. Blender is a pain in the ass and you have to be a very specific type of person to enjoy using it. Every hour you spend wrestling with Blender is an hour you're not actually making progress on your game and you'll burn out.